[media] mt9p031: Add support for regulators

Enable the regulators when powering the sensor up, and disable them when
powering it down.
The regulators are mandatory. Boards that don't allow controlling the
sensor power lines must provide fixed voltage regulators.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>