rtc: m41t80: add alarm functionality

Previous 'commit c3b79770 ("Expire alarms after the time is set")'
and 'commit 48e97667 ("remove disabled alarm functionality")' removed
the alarm support because the alarm irq was not functional.

Add the alarm IRQ functionality with newer functions than previous
code. Tested with 'rtctest' and the alarm is functional.

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