ext: nordic: Replace HAL and MDK with the ones from nrfx

The old HAL and MDK have been removed from the source tree.
Since RADIO HAL is not yet present in nrfx, the "nrf_radio.h" file
was temporarily moved to "nrfx/hal" folder. It will be replaced with
the proper file from nrfx in its next update.

Signed-off-by: Michał Kruszewski <michal.kruszewski@nordicsemi.no>
Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>