rtc: alarm: Add power-on alarm feature

Android does not support powering up of phone through alarm.
Adding shutdown hook in alarm driver which will set alarm while phone
is going down so as to power-up the phone after alarm expiration.

Change-Id: Ic2611e33ae9c1f8e83f21efdb93e26ac9f9499de
Signed-off-by: Matthew Qin <yqin@codeaurora.org>