OvmfPkg: IndustryStandard/Q35MchIch9.h: add PCIEXBAR macros

OvmfPkg: IndustryStandard/Q35MchIch9.h: add PCIEXBAR macros

Section 5.1.16 ("PCIEXBAR -- PCI Express Register Range Base Address") in
Intel document #316966-002 (already referenced near the top of this header
file) describes the Q35 DRAM Controller register that configures the
memory-mapped PCI config space (also known as MMCONFIG, and ECAM /
Enhanced Configuration Access Method).

In this patch we add the macros we'll need later. We'll only support the
256 MB memory-mapped config space -- enough for buses [0, 255].

Cc: Gabriel Somlo <somlo@cmu.edu>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: Michał Zegan <webczat_200@poczta.onet.pl>
Ref: https://github.com/tianocore/edk2/issues/32
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Tested-by: Gabriel Somlo <somlo@cmu.edu>
Tested-by: Michał Zegan <webczat_200@poczta.onet.pl>