Platform/RaspberryPi/RPi3: Add Device Tree driver

This driver serves the device tree that is either embedded
in the firmware volume or was provided through config.txt.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>