ASoC: add simple-mux

Add a driver for simple mux driven by gpios. It currently only supports one
gpio, muxing one of two inputs to a single output.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201205001508.346439-2-alexandre.belloni@bootlin.com


Signed-off-by: Mark Brown <broonie@kernel.org>