ENGR00209620 MX6-Disable PU Brown out detection in low power mode.

ENGR00209620 MX6-Disable PU Brown out detection in low power mode.

Ensure that the brown out detection of the PU regulator is
disabled before the regulator itself is disabled.
Keeping the detection enabled will generate an interrupt that
is not currently being handled in the BSP when the regulator is
disabled since its voltage is dropped to 0V. This will prevent
the system from entering complete WAIT state thus increasing the
power in the low power idle/audio usecase.

Signed-off-by: Ranjani Vaidyanathan <ra5478@freescale.com>