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

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

* changes:
  rpi4: Add initial documentation file
  rpi4: Add stdout-path to device tree
  rpi4: Add GIC maintenance interrupt to GIC DT node
  rpi4: Cleanup memory regions, move pens to first page
  rpi4: Reserve resident BL31 region from non-secure world
  rpi4: Amend DTB to advertise PSCI
  rpi4: Determine BL33 entry point at runtime
  rpi4: Accommodate "armstub8.bin" header at the beginning of BL31 image
  Add basic support for Raspberry Pi 4
  rpi3: Allow runtime determination of UART base clock rate
  FDT helper functions: Respect architecture in PSCI function IDs
  FDT helper functions: Add function documentation