ArmPkg,ArmPlatformPkg: Allow dynamic PCDs for memory base and size

ArmPkg,ArmPlatformPkg: Allow dynamic PCDs for memory base and size

This changes the definition and a bunch of references to
gArmTokenSpaceGuid.PcdSystemMemoryBase and
gArmTokenSpaceGuid.PcdSystemMemorySize so they can be declared as dynamic PCDs
by the platform. Also, move the non-SEC call to
ArmPlatformInitializeSystemMemory() earlier, so a platform has a chance to set
these PCDs before they are first referenced.

The purpose is allowing dynamically instantiated virtual machines to declare
the system memory by passing a device tree.

Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16079 6f19259b-4bc3-4df7-8a09-765794883524