MLK-13362 ARM: imx: fix audio bus mode hang on imx6sx/ul/sll

MLK-13362 ARM: imx: fix audio bus mode hang on imx6sx/ul/sll

When MMDC runs at a low frequency, it is not recommended to
perform "force measurement", the MMDC measure unit may return
a wrong measurement value when running below 100MHz.

Additionally, the double MU count operations should be only done
when changing the MMDC frequency from 400MHz to a low
frequency(100MHz or 24MHz). Otherwise, the MU count may overflow
and lead to system hang issue.

Signed-off-by: Bai Ping <ping.bai@nxp.com>