arm: mach-k3: am625: Fixup a53 cpu frequency by speed grade

The maximum frequency of the A53 CPU on the AM62 depends on the speed
grade of the SoC. However, this value is hardcoded in the DT for all
AM62 variants, potentially causing specifications to be exceeded. Moreover,
setting a common lower frequency for all variants increases boot time.
To prevent these issues, modify the DT at runtime from the R5 core to
adjust the A53 CPU frequency based on its speed grade.

Suggested-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Joao Paulo Goncalves <joao.goncalves@toradex.com>