rtc-at32ap700x: Enable wakeup

Call device_init_wakeup() to signal that the RTC is capable of waking
the system. This is needed for rtcwake to work.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Cc: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>