ArmVirtPkg/HighMemDxe: allow patchable PCD for PcdSystemMemoryBase

ArmVirtPkg/HighMemDxe: allow patchable PCD for PcdSystemMemoryBase

Redefine the reference to PcdSystemMemoryBase in HighMemDxe.inf as
a plain [Pcd] rather than [FixedPcd] (and fix up the code as
appropriate). This allows us to align ArmVirtQemuKernel with
ArmVirtQemu, given that the former uses a patchable PCD not a fixed
PCD.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>