soc: arm: nrf52: Update power management code for nRF52

This commit updates the power management code for nRF52 SoC
series after recent changes in Zephyr power management subsystem.

Changes:
- Removed LPS modes, as they are not supported by HW.
  (previous implemntation used CONSTLAT and LOWPWR settings
  in NRF_POWER to implement low power modes, however these
  knobs should be used only when application needs ultra low
  CPU wake-up time).

- Reworked code in order to allow adding custom power levels
  on the application level and provide better integration
  with power management subsystem.

Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>