rtc: m41t80: add wakealarm functionality

To enable the wakealarm, the device must be able to wakeup.
This is done by setting the device wakeup capability to true with
'device_init_wakeup' function.

Signed-off-by: Mylène Josserand <mylene.josserand@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>