ARM: at91/dt: add minimal sama5d2 Xplained board

Add minimal support for the new sama5d2 Xplained board. Only USB,
spi/i2c, ethernet and uart/usart peripherals added.
With this DTS file you can boot the board and begin to play with it.
Rootfs on NFS and sd card have successfully been tested.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>