Platform/RaspberryPi/RPi3: Add platform library

A platform helper library, relying on low level Mailbox messaging
between the CPU and VideoCore to obtain current platform information,
that is used by various services and drivers.

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>