ARM: atmel: add sama5d3 Xplained board support

Add sama5d3 Xplained board support which use Atmel SAMA5D36 SoC.
Now it supports boot from NAND flash and SD/MMC card.
Features support:
  - NAND flash
  - SD/MMC card
  - Two USB hosts
  - Ethernet (one GMAC, one EMAC)

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