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: I64640278f2efb4d8922df1fc7030d4f4bca9a528
Signed-off-by: Guixiong Wei <guixiong@codeaurora.org>