[media] adv7604: Support hot-plug detect control through a GPIO

Add support for optional GPIO-controlled HPD pins in addition to the
ADV7604-specific hotplug notifier.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>