Platform/RaspberryPi/RPi3: Add base MMC driver

This implements the MMC Host protocol, which is used by the two
concurrent SD interface drivers that the Raspberry Pi 3 supports.

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>