ArmPlatformPkg/PrePi: Make dynamic the top of the System Memory

ArmPlatformPkg/PrePi: Make dynamic the top of the System Memory

This change allows to change the top of the System Memory that
was hardcoded by:
FixedPcdGet64 (PcdSystemMemoryBase) + FixedPcdGet64 (PcdSystemMemorySize)

It allows to add support when the Trusted Firmware reserves the
top of the System Memory as Trusted. The size of this region
might not be known in advance.

Note: The reason why the start of the System Memory has not been
made dynamic is because the early code calculates where to place
the stack from the top of the System Memory.
So there is no need to make the start of the System Memory a
dynamic value at the early stage of the boot phase.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <Olivier.Martin@arm.com>
Reviewed-by: Ronald Cron <Ronald.Cron@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17835 6f19259b-4bc3-4df7-8a09-765794883524