[media] mt9p031: Implement V4L2_CID_PIXEL_RATE control

The pixel rate control is required by the OMAP3 ISP driver and should be
implemented by all media controller-compatible sensor drivers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>