ARM: at91/dt: enable the RTT block on the at91sam9m10g45ek board

Enable the RTT and GPBR devices and specify the general purpose register
used to store the current time.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>