rtc: pm8xxx: Read ALARM_EN and update to alarm enabled status

ALARM_EN status is retained in PMIC register after device shutdown
if poweron_alarm is enabled. Read it to make sure the driver has
consistent value with the register status.

Signed-off-by: Guixiong Wei <guixiong@codeaurora.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/1608809337-18852-1-git-send-email-guixiong@codeaurora.org