arm: mach-k3: am62: Get a53 max cpu frequency by speed grade

AM62 SoC has multiple speed grades. Add function to return max A53 CPU
frequency based on grade. Fastest grade's max frequency also depends on
PMIC voltage, to simplify implementation use the smaller value.

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