rtc: imx-sc: remove .read_alarm

The RTC core properly handles RTC without .read_alarm and doesn't use it to
set alarms. .read_alarm can be safely dropped.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20210417235205.994119-1-alexandre.belloni@bootlin.com