MLK-13798: rtc: snvs: add a missing write sync

MLK-13798: rtc: snvs: add a missing write sync

The clear of the LPTA_EN flag should be synced before writing to the
alarm register. Omitting this synchronization creates a race when
trying to change existing alarm.

(cherry picked from commit 7bb633b1a9812a6b9f3e49d0cf17f60a633914e5)

Signed-off-by: Guy Shapiro <guy.shapiro@mobi-wize.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Tested-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Octavian Purdila <octavian.purdila@nxp.com>