rtc: mt6397: enable wakeup before registering rtc device

rtc_sysfs_add_device checks if device can wakeup before creating the
wakealarm file in sysfs. Thus the driver must set wakeup capability
before registering the rtc device.

Signed-off-by: Wei-Ning Huang <wnhuang@google.com>
Acked-by: Eddie Huang <eddie.huang@mediatek.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>