drivers: regulator: add mode specific configuration functions for PMIC

Add mode specific PMIC configuration functions, to allow consumers to
configure target voltages of each regulator mode. This will allow users
to enable or disable, as well as set target voltages for regulator modes
without actually entering that mode.

This feature can be useful for power managment applications where the
consumer may want to switch the regulator to low power mode at a later
time.

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