ARM: at91/dt: at91sam9m10g45ek add rtc node

Add rtc node to both the at91sam9g45 SoC family and the at91sam9m10g45ek board.

Signed-off-by: Erik van Luijk <evanluijk@interact.nl>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>