Marvell/Armada70x0Db: Introduce board description library

This patch implements ArmadaBoarDescLib library for
Armada7040 Development Board and add to it ArmadaBoardDescSdMmcGet
function with description of connected Xenon host controllers.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>