The pixelclock detection of the adv7842 is precise enough to detect
if the framerate is 60 Hz or 59.94 Hz (aka "reduced fps").

Implement this detection.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>