drivers: regulator: add parent DVS API

PCA9420 PMIC offers of multiple operation states, or DVS (Dynamic
Voltage Scaling). Such states may be automatically changed by hardware
using MODESEL0/1 pins. Certain MCUs allow to automatically configure
certain output pins when entering low power modes so that PMIC state is
changed without software intervention.  This means that application just
needs to configure the voltages for each state using
`nxp,modeN-microvolt`, set `nxp,enable-modesel-pins` in devicetree and
forget about configuring regulators.

This patch introduces a new _parent_ API to expose such functionality in
a vendor agnostic way. Consider this API as experimental for now, until
we have other usecases.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>