Bluetooth: controller: Use nRFx functions for PPI reg access

In the BLE controller, NRF radio HAL, for the PPI registers used
for the SW TIFS.
To allow easier testing, replace direct use of registers
from Nordic's nrfx MDK with accesses via its HAL inlined functions

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>