arm64: dts: imx8mm: Add cpu speed grading and all OPPs

Add a nvmem cell on cpu node referencing speed grade and the 1.8 Ghz
cpufreq opp.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>