include: dt-bindings: update regulator defintions for PCA9420 for modes

Update regulator binding definitions for PCA9420 to support mode setting
features. This will allow the 4 voltage modes present on the PCA9420 PMIC
to be utilized.

Note that bindings have also been updated to indicate the default
register value and target voltage for each regulator on the PCA9420.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>