Encode soc name in the compatible string to know the specific version
hardware block.  This is the general approach adopted for most bindings.
Change mxs-pwm binding to use the approach.
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>
Signed-off-by: NThierry Reding <thierry.reding@avionic-design.de>