boards: shields: npm1300_ek: Settings moved to board overlay

The existing npm1300_ek shield overlay has several example
voltage and gpio settings.  For many use cases these are
invalid and need to be overridden, or removed with
delete-property.

These example configurations have been moved to a board
specific overlay.

Signed-off-by: Andy Sinclair <andy.sinclair@nordicsemi.no>