arm64: dts: imx8mm: Add cpufreq properties

This is very similar to imx8mq cpufreq-dt support.

Operating points are from datasheet:
https://www.nxp.com/docs/en/data-sheet/IMX8MMCEC.pdf



Higher opps were omitted (just like imx8mq) because it requires checking
speed grade from OCOTP fuses.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>