ASoC: imx: move audmux driver into sound/soc/imx

As audmux becomes a platform driver and its callers are all ASoC
machine drivers, there is no reason to keep it in arch folder, so
move it to sound/soc/imx.

One bonus point would be those ASoC machine drivers stop including
mach/audmux.h, since it's been moved to sound/soc/imx/imx-audmux.h.
This should be a move to the right direction in terms of single kernel
image goal.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>