rtc: qpnp-rtc: 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: Iee0a19ba5126265b36a353c1d1b249d09185564a
Signed-off-by: Mao Jinlong <c_jmao@codeaurora.org>