arm: atmel: Add SAMA5D2 Xplained board

The board supports following features:
 - Boot media support: SD card/e.MMC/SPI flash,
 - Support LCD display (optional, disabled by default),
 - Support ethernet,
 - Support USB mass storage.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
[fix checkpatch warnings]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>