ARM: imx: convert audmux to a platform driver

It coverts audmux to a platform driver, so that it can be moved into
sound/soc/imx and adopt device tree support later.

Signed-off-by: Richard Zhao <richard.zhao@linaro.org>
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>