PCI: Add PCI_EXP_LNKCTL2_TLS* macros

The Link Control 2 register is missing macros for Target Link Speeds.  Add
those in.

Signed-off-by: Frederick Lawler <fred@fredlawl.com>
[bhelgaas: use "GT" instead of "GB"]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>