colibri_t20: disable PMIC sleep mode on low supply voltage

The Colibri T20's PMIC enters a sleep mode on low supply voltage < 3.0V
±2.5% (2.92...3.08V). Rising the main supply voltage again does not
bring it back to regular operation. Not even a full reset does bring
the module back. A full power cycle was required to reboot the system.
A long positive pulse on the PMICs resume pin also reboots the system
but this pin is only accessible as a test point on the module.

This patch configures the PMIC through I2C to not enter this sleep mode
plus force it to normal state upon sleep request exit should this ever
happen.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>