ARM: at91/Documentation: add a README for Atmel SoCs

Add a README file to describe Atmel SoCs (aka AT91) support in Mainline Linux:
- SoC list + datasheet web links
- Basic but useful information
- Device Tree conventions and Work In Progress statement.
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Olof Johansson <olof@lixom.net>

Documentation/arm/Atmel/README

+ARM Atmel SoCs (aka AT91)
+=========================
+
+
+Introduction
+------------
+This document gives useful information about the ARM Atmel SoCs that are
+currently supported in Linux Mainline (you know, the one on kernel.org).
+
+It is important to note that the Atmel | SMART ARM-based MPU product line is
+historically named "AT91" or "at91" throughout the Linux kernel development
+process even if this product prefix has completely disappeared from the
+official Atmel product name. Anyway, files, directories, git trees,
+git branches/tags and email subject always contain this "at91" sub-string.
+
+
+AT91 SoCs
+---------
+Documentation and detailled datasheet for each product are available on
+the Atmel website: http://www.atmel.com.
+
+  Flavors:
+    * ARM 920 based SoC
+      - at91rm9200
+        + Datasheet
+          http://www.atmel.com/Images/doc1768.pdf
+
+    * ARM 926 based SoCs
+      - at91sam9260
+        + Datasheet
+          http://www.atmel.com/Images/doc6221.pdf
+
+      - at91sam9xe
+        + Datasheet
+          http://www.atmel.com/Images/Atmel-6254-32-bit-ARM926EJ-S-Embedded-Microprocessor-SAM9XE_Datasheet.pdf
+
+      - at91sam9261
+        + Datasheet
+          http://www.atmel.com/Images/doc6062.pdf
+
+      - at91sam9263
+        + Datasheet
+          http://www.atmel.com/Images/Atmel_6249_32-bit-ARM926EJ-S-Microcontroller_SAM9263_Datasheet.pdf
+
+      - at91sam9rl
+        + Datasheet
+          http://www.atmel.com/Images/doc6289.pdf
+
+      - at91sam9g20
+        + Datasheet
+          http://www.atmel.com/Images/doc6384.pdf
+
+      - at91sam9g45 family
+        - at91sam9g45
+        - at91sam9g46
+        - at91sam9m10
+        - at91sam9m11 (device superset)
+        + Datasheet
+          http://www.atmel.com/Images/Atmel-6437-32-bit-ARM926-Embedded-Microprocessor-SAM9M11_Datasheet.pdf
+
+      - at91sam9x5 family (aka "The 5 series")
+        - at91sam9g15
+        - at91sam9g25
+        - at91sam9g35
+        - at91sam9x25
+        - at91sam9x35
+        + Datasheet (can be considered as covering the whole family)
+          http://www.atmel.com/Images/Atmel_11055_32-bit-ARM926EJ-S-Microcontroller_SAM9X35_Datasheet.pdf
+
+      - at91sam9n12
+        + Datasheet
+          http://www.atmel.com/Images/Atmel_11063_32-bit-ARM926EJ-S-Microcontroller_SAM9N12CN11CN12_Datasheet.pdf
+
+    * ARM Cortex-A5 based SoCs
+      - sama5d3 family
+        - sama5d31
+        - sama5d33
+        - sama5d34
+        - sama5d35
+        - sama5d36 (device superset)
+        + Datasheet
+          http://www.atmel.com/Images/Atmel-11121-32-bit-Cortex-A5-Microcontroller-SAMA5D3_Datasheet.pdf
+
+    * ARM Cortex-A5 + NEON based SoCs
+      - sama5d4 family
+        - sama5d41
+        - sama5d42
+        - sama5d43
+        - sama5d44 (device superset)
+        + Datasheet
+          http://www.atmel.com/Images/Atmel-11238-32-bit-Cortex-A5-Microcontroller-SAMA5D4_Datasheet.pdf
+
+
+Linux kernel information
+------------------------
+Linux kernel mach directory: arch/arm/mach-at91
+MAINTAINERS entry is: "ARM/ATMEL AT91RM9200 AND AT91SAM ARM ARCHITECTURES"
+
+
+Device Tree for AT91 SoCs and boards
+------------------------------------
+All AT91 SoCs are converted to Device Tree. Since Linux 3.19, these products
+must use this method to boot the Linux kernel.
+
+Work In Progress statement:
+Device Tree files and Device Tree bindings that apply to AT91 SoCs and boards are
+considered as "Unstable". To be completely clear, any at91 binding can change at
+any time. So, be sure to use a Device Tree Binary and a Kernel Image generated from
+the same source tree.
+Please refer to the Documentation/devicetree/bindings/ABI.txt file for a
+definition of a "Stable" binding/ABI.
+This statement will be removed by AT91 MAINTAINERS when appropriate.
+
+Naming conventions and best practice:
+- SoCs Device Tree Source Include files are named after the official name of
+  the product (at91sam9g20.dtsi or sama5d33.dtsi for instance).
+- Device Tree Source Include files (.dtsi) are used to collect common nodes that can be
+  shared across SoCs or boards (sama5d3.dtsi or at91sam9x5cm.dtsi for instance).
+  When collecting nodes for a particular peripheral or topic, the identifier have to
+  be placed at the end of the file name, separated with a "_" (at91sam9x5_can.dtsi
+  or sama5d3_gmac.dtsi for example).
+- board Device Tree Source files (.dts) are prefixed by the string "at91-" so
+  that they can be identified easily. Note that some files are historical exceptions
+  to this rule (sama5d3[13456]ek.dts, usb_a9g20.dts or animeo_ip.dts for example).