PCI: Add #defines for accessing PCIe DVSEC fields

Add #defines for accessing Vendor ID, Revision, Length, and ID offsets
in the Designated Vendor Specific Extended Capability (DVSEC). Defined
in PCIe r5.0, sec 7.9.6.

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Link: https://lore.kernel.org/r/20211208015015.891275-2-david.e.box@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>