ARM: dts: at91: sama5d2: add shutdown controller node

Add the SAMA5D2-Compatible Shutdown Controller node to sama5d2.dtsi
and the use of it in the sama5d2 Xplained board dts file.

Enable the RTC wakeup event and the "wake up" button support through the
input "0" that is present on the board.

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