drivers: adc: nrf_saadc: add pm device support

Extend nrf saadc device driver with pm device runtime support.

To preserve previous behavior:

* if pm device is disabled, saadc is resumed on sampling start
  and suspended when sampling done.
* if pm device is enabled only, saadc does nothing on
  sampling start/stop. its resumed on init.
* if pm device runtime is enabled, saadc is got on
  sampling start, and put on sampling stop.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>