ARM: dts: at91: sama5d2 Xplained: add user push button

Add the push button named "PB USER" with code 0x104. Associated pinctrl node is
also added.

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