Platform/ARM/Sgi: Add initial support for SGI-Clark.Helios platform

Add the initial support for ARM's System Guidance for Infrastructure
SGI-Clark.Helios platform. The ACPI tables for this platform are
installed.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>