media: ov5640: Add HBLANK control

Add the HBLANK control as read-only.

The hblank value is fixed in the mode definition and is updated
everytime a new format is applied.

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>