ARM: at91/dt: sama5d2 Xplained: add device pin muxing

Add device pin muxing for the sama5d2 Xplained board.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>