ARM: at91/soc: add basic support for new sama5d2 SoC

Add Kconfig entries, header file changes and addition to the documentation.
The early debug infrastructure is also added for easy development.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Olof Johansson <olof@lixom.net>