Merge changes from topic "raspberry-pi-4-support" into integration

* changes:
  rpi3: Do prescaler and control setup in C
  rpi3: Prepare for supporting a GIC (in RPi4)
  rpi3: Make SHARED_RAM optional
  rpi3: Rename RPI3_IO_BASE to RPI_IO_BASE
  rpi3: Move shared rpi3 files into common directory