bluetooth: controller: abstract PPI handling functions

This commit refactors radio.c, so it calls abstract functions
for PPI handling (e.g. enable, disable channels, or configure
endpoints), which, then, call the platform-specific functions
for PPI handling. The abstract functions are simple wrappers,
implemented in radio_nrf5_ppi.h.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>