cpufreq: Add imx-cpufreq-dt driver

Right now in upstream imx8m cpufreq support just lists a common subset
of OPPs because the higher ones should only be attempted after checking
speed grading in fuses.

Add a small driver which checks speed grading from nvmem cells before
registering cpufreq-dt.

This driver allows unlocking all frequencies for imx8mm and imx8mq and
could be applied to other chips like imx7d

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>