MdePkg/IndustryStandard: introduce EFI_PCI_CAPABILITY_ID_SHPC

MdePkg/IndustryStandard: introduce EFI_PCI_CAPABILITY_ID_SHPC

The "Pci22.h" header file defines the macro EFI_PCI_CAPABILITY_ID_HOTPLUG
with value 0x06. According to all of:
- later parts of the same header file,
- Appendix H ("Capability IDs") of the PCI Local Bus Specification
  Revision 2.3,
- and Chapter 2 ("Capability IDs") of the PCI Code and ID Assignment
  Specification Revision 0.9,

0x06 means "CompactPCI Hot Swap". It does not mean "PCI Hot-Plug": that
capability is described by ID 0x0C:

  0Ch  PCI Hot-Plug -- This Capability ID indicates that the associated
       device conforms to the Standard Hot-Plug Controller model.

Therefore EFI_PCI_CAPABILITY_ID_HOTPLUG is arguably a misnomer. PciBusDxe
(mis-)uses EFI_PCI_CAPABILITY_ID_HOTPLUG in the IsSHPC() helper function
to identify PCI Hot-Plug capability.

In order to preserve compatibility with existent code, leave
EFI_PCI_CAPABILITY_ID_HOTPLUG alone, and introduce
EFI_PCI_CAPABILITY_ID_SHPC with the right ID value.

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: Liming Gao <liming.gao@intel.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: Michael Kinney <michael.d.kinney@intel.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>
Reviewed-by: Ruiyu Ni <Ruiyu.ni@intel.com>