mod_timer: Add clarifying comments

The timer module depends on the timer driver to enable and
disable the timer's interrupt source, however the timer module is
the one that clears the interrupt flag.

This behavior is explained in the timer driver API to avoid
ambiguity for timer driver developers.

Signed-off-by: Ahmed Gadallah <ahmed.gadallah@arm.com>
Change-Id: I982a9bffd3618c6b514e280e4601d4a58a9ce34a