Added missed clkmux setting for mx28evk.

Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Liam Girdwood <lrg@ti.com>
Cc: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>