powerpc: Add cpu family documentation

This patch adds some documentation on the different cpu families
supported by arch/powerpc.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

Documentation/powerpc/cpu_families.txt

+CPU Families
+============
+
+This document tries to summarise some of the different cpu families that exist
+and are supported by arch/powerpc.
+
+
+Book3S (aka sPAPR)
+------------------
+
+ - Hash MMU
+ - Mix of 32 & 64 bit
+
+   +--------------+                 +----------------+
+   |  Old POWER   | --------------> | RS64 (threads) |
+   +--------------+                 +----------------+
+          |
+          |
+          v
+   +--------------+                 +----------------+      +------+
+   |     601      | --------------> |      603       | ---> | e300 |
+   +--------------+                 +----------------+      +------+
+          |                                 |
+          |                                 |
+          v                                 v
+   +--------------+                 +----------------+      +-------+
+   |     604      |                 |    750 (G3)    | ---> | 750CX |
+   +--------------+                 +----------------+      +-------+
+          |                                 |                   |
+          |                                 |                   |
+          v                                 v                   v
+   +--------------+                 +----------------+      +-------+
+   | 620 (64 bit) |                 |      7400      |      | 750CL |
+   +--------------+                 +----------------+      +-------+
+          |                                 |                   |
+          |                                 |                   |
+          v                                 v                   v
+   +--------------+                 +----------------+      +-------+
+   |  POWER3/630  |                 |      7410      |      | 750FX |
+   +--------------+                 +----------------+      +-------+
+          |                                 |
+          |                                 |
+          v                                 v
+   +--------------+                 +----------------+
+   |   POWER3+    |                 |      7450      |
+   +--------------+                 +----------------+
+          |                                 |
+          |                                 |
+          v                                 v
+   +--------------+                 +----------------+
+   |    POWER4    |                 |      7455      |
+   +--------------+                 +----------------+
+          |                                 |
+          |                                 |
+          v                                 v
+   +--------------+     +-------+   +----------------+
+   |   POWER4+    | --> |  970  |   |      7447      |
+   +--------------+     +-------+   +----------------+
+          |                 |               |
+          |                 |               |
+          v                 v               v
+   +--------------+     +-------+   +----------------+
+   |    POWER5    |     | 970FX |   |      7448      |
+   +--------------+     +-------+   +----------------+
+          |                 |               |
+          |                 |               |
+          v                 v               v
+   +--------------+     +-------+   +----------------+
+   |   POWER5+    |     | 970MP |   |      e600      |
+   +--------------+     +-------+   +----------------+
+          |
+          |
+          v
+   +--------------+
+   |   POWER5++   |
+   +--------------+
+          |
+          |
+          v
+   +--------------+       +-------+
+   |    POWER6    | <-?-> | Cell  |
+   +--------------+       +-------+
+          |
+          |
+          v
+   +--------------+
+   |    POWER7    |
+   +--------------+
+          |
+          |
+          v
+   +--------------+
+   |   POWER7+    |
+   +--------------+
+          |
+          |
+          v
+   +--------------+
+   |    POWER8    |
+   +--------------+
+
+
+   +---------------+
+   | PA6T (64 bit) |
+   +---------------+
+
+
+IBM BookE
+---------
+
+ - Software loaded TLB.
+ - All 32 bit
+
+   +--------------+
+   |     401      |
+   +--------------+
+          |
+          |
+          v
+   +--------------+
+   |     403      |
+   +--------------+
+          |
+          |
+          v
+   +--------------+
+   |     405      |
+   +--------------+
+          |
+          |
+          v
+   +--------------+
+   |     440      |
+   +--------------+
+          |
+          |
+          v
+   +--------------+     +----------------+
+   |     450      | --> |      BG/P      |
+   +--------------+     +----------------+
+          |
+          |
+          v
+   +--------------+
+   |     460      |
+   +--------------+
+          |
+          |
+          v
+   +--------------+
+   |     476      |
+   +--------------+
+
+
+Motorola/Freescale 8xx
+----------------------
+
+ - Software loaded with hardware assist.
+ - All 32 bit
+
+   +-------------+
+   | MPC8xx Core |
+   +-------------+
+
+
+Freescale BookE
+---------------
+
+ - Software loaded TLB.
+ - e6500 adds HW loaded indirect TLB entries.
+ - Mix of 32 & 64 bit
+
+   +--------------+
+   |     e200     |
+   +--------------+
+
+
+   +--------------------------------+
+   |              e500              |
+   +--------------------------------+
+                   |
+                   |
+                   v
+   +--------------------------------+
+   |             e500v2             |
+   +--------------------------------+
+                   |
+                   |
+                   v
+   +--------------------------------+
+   |        e500mc (Book3e)         |
+   +--------------------------------+
+                   |
+                   |
+                   v
+   +--------------------------------+
+   |          e5500 (64 bit)        |
+   +--------------------------------+
+                   |
+                   |
+                   v
+   +--------------------------------+
+   | e6500 (HW TLB) (Multithreaded) |
+   +--------------------------------+
+
+
+IBM A2 core
+-----------
+
+ - Book3E, software loaded TLB + HW loaded indirect TLB entries.
+ - 64 bit
+
+   +--------------+     +----------------+
+   |   A2 core    | --> |      WSP       |
+   +--------------+     +----------------+
+           |
+           |
+           v
+   +--------------+
+   |     BG/Q     |
+   +--------------+