alarm: clear pending interrupt when an alarm is stopped

If alarm_stop() is called while the interrupt are globally disabled,
it is possible that an interrupt is pending because the alarm being stopped
has triggered. To avoid the execution of the timer ISR after the alarm has
been stopped, this interrupt is cleared in the function.

Change-Id: I3bbce3aeb447546e541b3d6d453a16f5b4db9445
Signed-off-by: Elieva Pignat <Elieva.Pignat@arm.com>