rtc: isl12022: Add alarm support

The ISL12022 RTC has a combined INT/fOUT pin, which can be used for alarm
interrupt when frequency output is not enabled.

The device-tree bindings should ensure that interrupt and clock output is
not enabled at the same time.

Signed-off-by: Esben Haabendal <esben@geanix.com>
Link: https://lore.kernel.org/r/20240913-rtc-isl12022-alarm-irq-v2-2-37309d939723@geanix.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>