Add support for the alarms. The match on the weekday is not used as it it
not necessarily properly set.

The tested RTC shows a behaviour where setting an alarm on the second right
after an alarm that fired is not working, probably because of the circuit
that ensures an alarm only fires once. This is why uie_unsupported is set.
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>