drivers: regulator: improve regulator_list_voltages

- Rename to regulator_list_voltage (it is listing a single voltage)
- Function returns the value via a parameters, so that we can indicate
  wether the given index is valid or not. If a driver doesn't implement
  this call, function returns -EINVAL (as it should be).

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