media: platform: video-mux: convert to multiplexer framework

Now that the multiplexer framework is merged, drop the temporary
mmio-mux implementation from the video-mux driver and convert it to use
the multiplexer API.

[mchehab@s-opensource.com: fix a merge conflict at Kconfig]
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>