rtc: snvs: add a missing write sync

[ Upstream commit 7bb633b1a9812a6b9f3e49d0cf17f60a633914e5 ]

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.

Signed-off-by: Guy Shapiro <guy.shapiro@mobi-wize.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>