[media] add mux and video interface bridge entity functions

Add two new media entity function definitions for video multiplexers
and video interface bridges.

- renamed MEDIA_ENT_F_MUX to MEDIA_ENT_F_VID_MUX

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>