clocksource/drivers/timer-of: Add kernel documentation

clocksource/drivers/timer-of: Add kernel documentation

The current code has no comments, neither any function descriptions. Fix this by
adding function descriptions in kernel doc format.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1515418139-23276-6-git-send-email-daniel.lezcano@linaro.org
[ Spelling and style fixes. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>