Alarm: Use dedicated alarm type for poweroff alarm.

RTC_POWEROFF_WAKEUP is a new alarm type introduced by framework
to perform poweroff alarm. The time (RTC_WAKEUP) set to framework
is not the same as the alarm time set in deskclock.  In this case
the RTC register might not be correctly set. So both RTC_WAKEUP
and RTC_POWEROFF_WAKEUP types need to be set at the same time:
RTC_WAKEUP type will handle the normal alarm process, and
RTC_POWEROFF_WAKEUP is used to set the correct alarm time to RTC
register.

Change-Id: Ic03f87018e163adf6f6e6013963a497d340fbec7