ASoC: imx: rename audmux prefix mxc to imx

It renames the legacy name mxc used in audmux function and macro
to imx.

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>