ntb: intel: add Intel NTB LTR vendor support for gen4 NTB

Intel NTB device has custom LTR management that is not compliant with the
PCIe standard. Add support to set LTR status triggered by link status
change.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>