[media] media: ov7670: Add possibility to disable pixclk during hblank

Some bridge drivers capture pixels during blanking periods if
pixclk is enabled. In order to avoid capturing bogus data we need to
disable pixclk in the sensor during those blanking periods.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>