rtc: pcf2123: implement .alarm_irq_enable

Implement .alarm_irq_enable so it is possible to use RTC_ALM_SET,
RTC_AIE_ON and RTC_AIE_OFF.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20190819182656.29744-3-alexandre.belloni@bootlin.com


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