rtc: m41t93: Add alarm support

rtc: m41t93: Add alarm support

Implement alarm feature for rtc-m41t93 by adding necessary
callbacks - set_alarm, read_alarm and alarm_irq_enable.
Enable support to configure alarm 1 out of 2 alarms present in this rtc.
Support only alarm configuration in this commit. This commit does not
implement alarm irq handling.

Use selftests/rtc/rtctest for testing. Tested by observing IRQ pin
(pin 12 of SOX18 package) on logic analyzer going low after alarm
condition is met.

Signed-off-by: Akhilesh Patil <akhilesh@ee.iitb.ac.in>
Link: https://patch.msgid.link/1b272ff1a5392d5eb76e129a4785ac8424763356.1758379856.git.akhilesh@ee.iitb.ac.in
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>