imx8mm: Update CPU speed grading

According to iMX8MM datasheet (IMX8MMIEC_Rev_D and IMX8MMCEC_Rev_D),
the speed grading for imx8mm is 800Mhz, 1.2Ghz, 1.6Ghz and 1.8Ghz.
Update them to get_cpu_speed_grade_hz function.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
1 file changed