karo: tx6ul: disable the PMIC for '_noenv' U-Boot

The I2C access to the PMIC requires the TAMPER pins of the i.MX6UL to
be useable as GPIOs. This is only possible after the TAMPER_PIN_DISABLE
fuses are programmed which is usually done via U-Boot.
Resolve this catch 22 situation by disabling the PMIC in the '_noenv'
U-Boot variant which is usually used in the manufacturing
environment.