MdeModulePkg/PciBusDxe: recognize hotplug-capable PCIe ports
    
    Section 7.8.2 of the PCI Express specification (r4.0 v0.3), entitled "PCI
    Express Capabilities Register (Offset 02h)", describes the conditions when
    a PCIe port should be considered "supporting hotplug":
    - it should be a root complex port or a switch downstream port, and
    - it should have the "Slot Implemented" bit set.
    
    This logic is already implemented in at least two open source projects I
    could find:
    
    - in SeaBIOS by Marcel Apfelbaum: "hw/pci: reserve IO and mem for pci
      express downstream ports with no devices attached"
      <https://code.coreboot.org/p/seabios/source/commit/3aa31d7d6375>,
    
    - in edk2 itself, in the implementation of the "PCI" UEFI Shell command:
      see the "PcieExplainTypeSlot" case label in function
      PciExplainPciExpress(), file
      "ShellPkg/Library/UefiShellDebug1CommandsLib/Pci.c".
    
    PciBusDxe recognizes such PCIe ports as bridges, but it doesn't realize
    they support hotplug. In turn PciBusDxe omits getting any resource padding
    information from the platform's EFI_PCI_HOT_PLUG_INIT_PROTOCOL for these
    bridges:
    
      GatherPpbInfo()                [PciEnumeratorSupport.c]
        GetResourcePaddingPpb()      [PciResourceSupport.c]
          GetResourcePaddingForHpb() [PciHotPlugSupport.c]
            IsPciHotPlugBus()        [PciHotPlugSupport.c]
              //
              // returns FALSE
              //
            //
            // the following is not reached:
            //
            gPciHotPlugInit->GetResourcePadding()
    
    Implement a function called SupportsPcieHotplug() for identifying such
    ports, and call it from IsPciHotPlugBus() (after the call to IsSHPC()).
    
    Cc: "Johnson, Brian J." <bjohnson@sgi.com>
    Cc: Alex Williamson <alex.williamson@redhat.com>
    Cc: Andrew Fish <afish@apple.com>
    Cc: Feng Tian <feng.tian@intel.com>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Marcel Apfelbaum <marcel@redhat.com>
    Cc: Ruiyu Ni <ruiyu.ni@intel.com>
    Cc: Star Zeng <star.zeng@intel.com>
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>