media: ov5640: Add LINK_FREQ control

Add the V4L2_CID_LINK_FREQ control to the ov5640 driver.
Make the control read-only for the moment.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>