[media] rcar-vin: use monotonic timestamps

Even though the rcar-vin driver tells userspace that it will give a monotonic
timestamp, it is actually using gettimeofday. Replace this with a proper
monotonic timestamp.

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