OvmfPkg: factor the MMIO aperture shared by all PCI root bridges into PCDs

OvmfPkg: factor the MMIO aperture shared by all PCI root bridges into PCDs

Going forward, two modules will need to know about the aperture:
PlatformPei (as before), and OVMF's upcoming PciHostBridgeLib instance
(because the core PciHostBridgeDxe driver requires the library to state
the exact apertures for all root bridges).

On QEMU, all root bridges share the same MMIO aperture, hence one pair of
PCDs suffices.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>