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

i.MX8MN has different speed grading definition as below, it has 4 bits
to define speed grading, add support for it.

 SPEED_GRADE[3:0]    MHz
    0000            2300
    0001            2200
    0010            2100
    0011            2000
    0100            1900
    0101            1800
    0110            1700
    0111            1600
    1000            1500
    1001            1400
    1010            1300
    1011            1200
    1100            1100
    1101            1000
    1110             900
    1111             800

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