[media] v4l: omap4iss: Set the vb2 timestamp type

Timestamps use the monotonic clock, configure the vb2 queue accordingly.

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