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 (One MMC card slot, One TF card slot)
  - USB (two host ports, one USB device port)
  - Ethernet (one GMAC, one EMAC)

Signed-off-by: Bo Shen <voice.shen@atmel.com>
ARM: atmel: sama5d3 xplained: add mtest command

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>