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>
diff --git a/drivers/clocksource/timer-of.h b/drivers/clocksource/timer-of.h
index e0d7272..44f57e0 100644
--- a/drivers/clocksource/timer-of.h
+++ b/drivers/clocksource/timer-of.h
@@ -66,4 +66,7 @@ static inline unsigned long timer_of_period(struct timer_of *to)
 
 extern int __init timer_of_init(struct device_node *np,
 				struct timer_of *to);
+
+extern void timer_of_exit(struct timer_of *to);
+
 #endif