ARM: dts: at91: sama5d2_xplained: set PB_USER as wakeup source

Set the PB_USER button as a wakeup source to resume from ulp0 mode.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
[claudiu.beznea@microchip.com: change subject to match the desired prefix]
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>