[media] MAINTAINERS: add maintainer entry for video multiplexer v4l2 subdevice driver

Add maintainer entry for the video multiplexer v4l2 subdevice driver that
will control video bus multiplexers via the multiplexer framework.

Signed-off-by: Philip Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>