OvmfPkg: PlatformPei: lower the 32-bit PCI MMIO base to 2GB on Q35
    
    Gerd has advised us that long term support Q35 machine types have no low
    RAM above 2GB, hence we should utilize the [2GB, 3GB) gap -- that we
    currently leave unused -- for MMIO. (Plus, later in this series, for the
    PCIEXBAR too.)
    
    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
    Ref: http://thread.gmane.org/gmane.comp.bios.edk2.devel/8707/focus=8817
    Suggested-by: Gerd Hoffmann <kraxel@redhat.com>
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>
    Tested-by: Gabriel Somlo <somlo@cmu.edu>
    Tested-by: Michał Zegan <webczat_200@poczta.onet.pl>
    Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>