cpufreq: imx-cpufreq-dt: Correct i.MX8MN's default speed grade value

i.MX8MN has different speed grade definition compared to
i.MX8MQ/i.MX8MM, when fuses are NOT written, the default
speed_grade should be set to minimum available OPP defined
in DT which is 1.2GHz, the corresponding speed_grade value
should be 0xb.

Fixes: 5b8010ba ("cpufreq: imx-cpufreq-dt: Add i.MX8MN support")
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>