cpufreq: imx-cpufreq-dt: Add i.MX8MN support

i.MX8MN is a new SoC of i.MX8M series, it also uses speed
grading and market segment fuses for OPP definitions, add
support for this SoC.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>