Platforms/RPi3: Restructure platform in preparation for Pi 4

In preparation for adding Raspberry Pi 4 support, the Pi 3 platform
is restructured by factorizing all the drivers and libraries that are
going to be commonly used by the two platforms.

Because much of the Pi 4 SoC is an extension of the Pi 3 one this
means that almost everything, except the ACPI tables, is moved up
into a new common RaspberryPi/ subdirectory that will serve both
platforms. The .dec is also moved to this directory, under a new
RaspberryPi.dec name, and existing references to it are updated.

This commit requires the edk2-non-osi in use to contain commit 243e55f622ea
in order to build.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>