board: verdin-am62: set cpu core voltage depending on speed grade

Speed grade T requires the VDD_CORE voltage to be 0.85V if using
the maximum core frequency.

Speed grades G, K, S allow the VDD_CORE voltage to be 0.75V up to the
maximum core frequency but allow to run at 0.85V.

For efficiency in manufacturing and code maintenance we use 0.85V for
the PMIC defaults and device tree settings and dynamically adjust the
voltage in the PMIC and device tree to 0.75V for lower speed SKU to
gain more than 100mW power consumption reduction.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>