davinci: da8xx/omap-l1: mark RTC as a wakeup source

On da850, RTC alarm is a wakeup source from deep sleep.
Mark it as a wakeup source after the rtc platform device
is registered.

Without this patch, the rtc-omap driver suspends the RTC
during the suspend sequence and hence it cannot wakeup the
SoC.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>