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>