pci_regs.h: Add PCI bus link speed and width defines

Add missing PCI bus link speed 8.0 GT/s and bus link widths of
x1, x2, x4 and x8.

CC: <linux-kernel@vger.kernel.org>
CC: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>