drivers: regulator: pca9420: fixed child instantiation

Right now the PCA9420 driver instantiates by iterating over all
children. This is somewhat problematic, for a few reasons:

- Since instantiation is generic code, we're forced to put internal
  details on Devicetree, e.g. reg-masks. After this change, this will no
  longer be necessary.
- We take all children, regardless of what is defined in DT.

While we have no means to validate Devicetree node names as in Linux
dtschema, this approach allows us to have per-child specific
initialization code. This is somewhat similar to the Linux approach.

Note: nodelabels have been removed, since they were not used.

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