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
