rtc: 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.

Change-Id: Ie0c5ce67cf6ee39ee6f156d8b7a4a81c624adec1
Signed-off-by: Guixiong Wei <guixiong@codeaurora.org>