ArmVirtPkg/ArmVirtMemoryInitPeiLib: handle memory above 4 GB on 32-bit ARM

On 32-bit ARM, split system memory into a region below (and up to) 4 GB
and a region above 4 GB. This is necessary to get the DXE core to consider
the former as the resource descriptor that describes the primary memory
region that also covers the PHIT region.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18427 6f19259b-4bc3-4df7-8a09-765794883524