ARM: at91/dt: enable the RTT block on the sam9g20ek board

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

Enable wakeup on RTT event on the shutdown controller device.

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