rtc: alarm: set power_on_alarm again when calling alarmtimer_resume

The power_on alarm might be replaced or cleared during alarm
suspend and resume process.So restore the power_on alarm into RTC
during alarm resume.

Change-Id: I1eefb082f9dfbd78517f3da1cc60a989853555fd
Signed-off-by: Mao Jinlong <c_jmao@codeaurora.org>