ARM: at91: add SAMA5D3 Xplained board

Add DT file and defconfig file for new SAMA5D3 Xpained board.
This board is based on Atmel's SAMA5D36 Cortex-A5 SoC.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
ARM: at91: add phy fixup to board file for SAMA5D3 Xplained

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