[media] omap_vout: add missing OVERLAY_OUTPUT cap and set V4L2_FBUF_FLAG_OVERLAY

The omap_vout driver has an output overlay, but never advertised that
capability.

The driver should also set the V4L2_FBUF_FLAG_OVERLAY flag.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
CC: Archit Taneja <archit@ti.com>
CC: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>