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

mainline inclusion
from mainline-v5.11-rc1
commit 75b6f648
category: feature
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I5IZNI



Intel-SIG: commit 75b6f648
ntb: intel: add Intel NTB LTR vendor support for gen4 NTB.
Backport for Intel NTB LTR vendor support patch

-------------------------------------

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>
Signed-off-by: Zheng Wu <wu.zheng@intel.com>