powerpc: Add some documentation of ISA versions

Add some documentation on which CPU versions map to which ISA
versions. This is all publicly available information, some of it
already in the kernel source, but it's much nicer to have it all in
one place.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Documentation/powerpc/isa-versions.rst

+CPU to ISA Version Mapping
+==========================
+
+Mapping of some CPU versions to relevant ISA versions.
+
+========= ====================
+CPU       Architecture version
+========= ====================
+Power9    Power ISA v3.0B
+Power8    Power ISA v2.07
+Power7    Power ISA v2.06
+Power6    Power ISA v2.05
+PA6T      Power ISA v2.04
+Cell PPU  - Power ISA v2.02 with some minor exceptions
+          - Plus Altivec/VMX ~= 2.03
+Power5++  Power ISA v2.04 (no VMX)
+Power5+   Power ISA v2.03
+Power5    - PowerPC User Instruction Set Architecture Book I v2.02
+          - PowerPC Virtual Environment Architecture Book II v2.02
+          - PowerPC Operating Environment Architecture Book III v2.02
+PPC970    - PowerPC User Instruction Set Architecture Book I v2.01
+          - PowerPC Virtual Environment Architecture Book II v2.01
+          - PowerPC Operating Environment Architecture Book III v2.01
+          - Plus Altivec/VMX ~= 2.03
+========= ====================
+
+
+Key Features
+------------
+
+========== ==================
+CPU        VMX (aka. Altivec)
+========== ==================
+Power9     Yes
+Power8     Yes
+Power7     Yes
+Power6     Yes
+PA6T       Yes
+Cell PPU   Yes
+Power5++   No
+Power5+    No
+Power5     No
+PPC970     Yes
+========== ==================
+
+========== ====
+CPU        VSX
+========== ====
+Power9     Yes
+Power8     Yes
+Power7     Yes
+Power6     No
+PA6T       No
+Cell PPU   No
+Power5++   No
+Power5+    No
+Power5     No
+PPC970     No
+========== ====
+
+========== ====================
+CPU        Transactional Memory
+========== ====================
+Power9     Yes (* see transactional_memory.txt)
+Power8     Yes
+Power7     No
+Power6     No
+PA6T       No
+Cell PPU   No
+Power5++   No
+Power5+    No
+Power5     No
+PPC970     No
+========== ====================