rtc: m41t80: add alarm functionality

rtc: m41t80: add alarm functionality

Previous 'commit c3b79770e51a ("Expire alarms after the time is set")'
and 'commit 48e9766726eb ("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>