soc: nordic: Add support for nRF52820

This commit adds basic support for nRF52820 SoC.
Changes affect introducing:
 - architecuture files (dtsi)
 - configuration of nrfx drivers
 - adaptation of inclusions based on chosen SoC

Signed-off-by: Mieszko Mierunski <mieszko.mierunski@nordicsemi.no>