rtc: alarm: init power_on_alarm_lock mutex in alarmtimer_rtc_timer_init

Moved mutex_init of power_on_alarm_lock into alarmtimer_rtc_timer_init
so that if CONFIG_RTC_CLASS is not enabled, we dont try to initialize
the mutex.  This fixes a build issue when !CONFIG_RTC_CLASS

Change-Id: I270564b94fd54162aa177a2c9767655098662259
Signed-off-by: Kumar Gala <galak@codeaurora.org>