Platform/RPi4: Add base platform files

This enables building the initial RPi4 platform firmware.

Note that PCIe and xHCI are missing at this stage and that this
version of the firmware uses miniUART for serial I/O. PCIe and
xHCI support will be added in a later patch series as well as
the ability to switch between PL011 and miniUART for serial.

Details on how to use the resulting firmware, and especially how
to configure the media for boot, are provided in the Readme.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>