PCI: expose pcie_link_speed and pcix_bus_speed arrays

pcie_link_speed and pcix_bus_speed are arrays used by probe.c to correctly
convert lnksta register values into the pci_bus_speed enum. These static arrays
are useful outside probe for this purpose. This patch makes these defines into
conist arrays and exposes them with an extern header in drivers/pci/pci.h

-v2-
* move extern declarations to drivers/pci/pci.h

CC: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>