PCI: Add #defines for some of PCIe spec r4.0 features

Add #defines only for the Data Link Feature and Physical Layer 16.0 GT/s
features as defined in PCIe spec r4.0, sec 7.7.4 for Data Link Feature and
sec 7.7.5 for Physical Layer 16.0 GT/s.

Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>