ArmVirtPkg/HighMemDxe: check new regions against GCD memory space map

Instead of looking at the PCD gArmTokenSpaceGuid.PcdSystemMemoryBase
to decide which DT node covers the memory we are already using, query
the GCD memory space map, which is the authoritative source for this
kind of information

This fixes a problem observed by Michael on platforms where this PCD
is of the 'Patchable' type, which means updates to its value do not
propagate to other modules.

Reported-by: Michael Zimmermann <sigmaepsilon92@gmail.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>