[media] platform: add video-multiplexer subdevice driver

This driver can handle SoC internal and external video bus multiplexers,
controlled by mux controllers provided by the mux controller framework,
such as MMIO register bitfields or GPIOs. The subdevice passes through
the mbus configuration of the active input to the output side.

Since the mux framework is not yet merged, this driver contains
temporary mmio-mux support to work without the framework. The driver
should be converted to use the multiplexer API once the "mux: minimal
mux subsystem" and "mux: mmio-based syscon mux controller" patches are
merged.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
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>
Acked-by: Pavel Machek <pavel@ucw.cz>
[hans.verkuil@cisco.com: add 'select REGMAP' to Kconfig]
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>