[media] media: adv7180: add power pin control

Some targets control the ADV7180 power pin via a gpio, so add
optional support for "powerdown" pin control.

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Tested-by: Tim Harvey <tharvey@gateworks.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>