board: imx6ull: Add disable PMIC_STBY_REQ

Disable output driver of PAD CCM_PMIC_STBY_REQ. This prevents the
SOC to request for a lower voltage during sleep. This is necessary
because the voltage is changing too slow for the SOC to wake up
properly.

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@toradex.com>