dts: arm: nordic: nrf52x: instantiate regulators

The SoC main supply is part of the POWER IP block. The POWER IP is a
kind of multi-purpose block, so each of its functions is described as a
child node in DT, like similar other MFD. This allows to have specific
properties, e.g. for DC/DC mode.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>