Platform/ARM/Sgi: Install a Platform ID HOB

On SGI platforms, the trusted firmware executes prior to
the SEC phase. It supplies to the SEC phase a pointer to
a fdt, that contains platform specific information such as
part number and config number of the SGI platform. The
platform code that executes during the SEC phase creates a
PPI that allows access to other PEIMs to obtain a copy of the
fdt pointer.

Further, during the PEI phase, a Platform ID PEIM installs a
Platform ID HOB. The Platform ID HOB can be used by DXE
drivers to identify the platform it is executing on.

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