PCI/ASPM: Enable Latency Tolerance Reporting when supported

PCI/ASPM: Enable Latency Tolerance Reporting when supported

BugLink: http://bugs.launchpad.net/bugs/1756094
Enable Latency Tolerance Reporting (LTR).  Note that LTR must be enabled in
the Root Port first, and must not be enabled in any downstream device
unless the Root Port and all intermediate Switches also support LTR.
See PCIe r3.1, sec 6.18.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Vidya Sagar <vidyas@nvidia.com>
(cherry picked from commit c46fd358070f22ba68d6e74c22016a33b914c20a)
Signed-off-by: Manoj Iyer <manoj.iyer@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>