clocksource/drivers/timer-of: Add timer_of_exit function

The timer-of API does not provide a function to undo what has been done by
the timer_of_init() function.

Add a timer_of_exit() function.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>