drivers: regulator: add i2c regulator driver

This commit adds a generic i2c regulator driver, and enables the NXP
PCA9420 PMIC IC using this driver. The regulator driver also exposes an
additional API in include/drivers/regulator/consumer.h, which allows
drivers to implement support for adjusting voltage levels and current
limits, if their device supports it.

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