boards: npm6001_ek: add regulator entries

Add entries for the BUCK0/1/2/3 and LDO0/1 regulators. Note that voltage
min/max limits have been set to the hardware limits. They can be
adjusted using overlays in applications, depending on the consumer
circuit constraints.

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