ASoC: fsl: add dt support for imx-audmux

It adds device tree probe support for imx-audmux driver.

Signed-off-by: Richard Zhao <richard.zhao@linaro.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>