media: adv7180: add g_skip_frames support

The adv7180 produces 1 to 2 frames of garbage before proper sync is
established. This allows V4L2 drivers and apps to skip those.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>