ARM: atmel: add sama5d4 xplained ultra board support

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

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>