cpufreq: imx-cpufreq-dt: Add i.MX8MP support

Add i.MX8MP cpufreq DT support for speed grading and market
segment check.

Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
[ Viresh: Minor formatting fixes ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>