[media] v4l: xilinx: Add Video Timing Controller driver

The Video Timing Controller (VTC) includes a timing detector and/or a
timing generator. Only the generator is currently supported.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>