pm: improve logging

List of improvements:

- The PM logging module was only available if CONFIG_PM=y, however, it
  was also used by Device PM (which can be selected without PM). A new
  logging module has been created for Device PM.
- Log level is passed to LOG_MODULE_(DECLARE|REGISTER)
- Logger name has been adjusted to `pm` (was `power`)

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