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

This reverts commit d2099587.

According to TI changing the VDD_CORE while the SoC is running is not
allowed, the voltage must be set before the AM62 device reset is
released, revert this change therefore.

The correct solution would be to program the PMIC during manufactoring
according to the speed grade of the SoC.

Link: https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1318338/am623-booting-from-mmc-failed-after-lowering-vdd_core-to-0-75v/5036508#5036508


Fixes: d2099587 ("board: verdin-am62: set cpu core voltage depending on speed grade")
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>