timer: alarm triggers a callback function

When an alarm is triggered a callback function is called instead
of sending an event to the module or element that set the alarm.

That way an alarm trigger does not necessarily generate an event
and for example register polling can be done more effectively.

Change-Id: I7ef16976c8f90e3f772ae57056f5891636533170
Signed-off-by: Elieva Pignat <Elieva.Pignat@arm.com>