PCI/ASPM: Enable Latency Tolerance Reporting when supported

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>

include/linux/pci.h

@@ -350,6 +350,8 @@ struct pci_dev {
 
 #ifdef CONFIG_PCIEASPM
 	struct pcie_link_state	*link_state;	/* ASPM link state */
+	unsigned int	ltr_path:1;	/* Latency Tolerance Reporting
+					   supported from root to here */
 #endif
 
 	pci_channel_state_t error_state;	/* current connectivity state */