[media] adv7180: Add support for power down

The adv7180 has a low power mode in which the analog and the digital processing
section are shut down. Implement the s_power callback to let bridge drivers put
the part into low power mode when not needed.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>