ARM: mxs: Setup scheduler clock

Setup scheduler clock on ARM MXS platforms with a 32-bit timrot
such as i.MX28. This allows the scheduler to use sub-jiffy resolution.

The corresponding change for 16-bit v1 timrots is not possible
at the moment due to rounding issues with clock values wrapping
faster than once per several seconds in the common ARM platform code.

Signed-off-by: Stanislav Meduna <stano@meduna.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>