Platforms/RPi3: Add multiple embedded Device Tree selection

The Raspberry Pi 3 platform currently has 2 different models, each with a
different Device Tree. Rather than embedding a single one, and requiring
users to manually provide the other, this patch ensures that we now embed
both and and serve the relevant one at runtime.

This patch also adds support for the Raspberry Pi 4 in FdtDxe.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>