arm: mach-k3: am62p: Set a53 cpu freq based on speed-grade

arm: mach-k3: am62p: Set a53 cpu freq based on speed-grade

The maximum frequency of the A53 CPU on the AM62P depends on the speed
grade of the SoC. This value is hardcoded in the DT for all AM62P
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.

Signed-off-by: Aparna Patra <a-patra@ti.com>
Reviewed-by: Bryan Brattlof <bb@ti.com>