arm64: dts: kontron-mx8mm: Remove incorrect regulator 'regulator-boot-on' properties

Remove the 'regulator-boot-on' on flags. They mean that the reulator
was turned on in the bootloader/firmware, which in these cases is
either not true, or not applicable for regulators without on/off
control.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>