drivers: regulator: add support to PMIC regulator driver for modes

Add support for setting the target mode for a PMIC regulator. Some
regulators support multiple modes, each with distinctive voltage and
current configuration data. This function allows the consumer to switch
the PMIC into a new mode. The PMIC can then be configured to use a new
set of voltages.

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