drivers: regulator: pca9420: do not expose registers in DT

Regulator registers were set for each BUCK/LDO in DT, likely because of
the way the devices were instantiated. When using a generic iterator,
ie, DT_INST_FOREACH_CHILD, there's no way to differentiate the child
being _parsed_. Since instantiation happens now based on child node
names, we are able to know which registers each devices gets assigned at
the driver level. This greatly simplifies Devicetree, and it actually
removes information that is not strictly hardware description from it.

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