PCIe: Add L1 Sub-State support.

Enable L1 Sub-State when both root port and endpoint support it.

[pg: keyed the feature to MMCONF_SUPPORT, otherwise boards
without that capability fail to build.]

Change-Id: Id11fc7c73eb865411747eef63f5f901e00a17f84
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 6ac04ad7e2261846e40da297f7fa317ccebda092
Original-BUG=chrome-os-partner:31424
Original-TEST=Build a image and run on Samus proto boards to check if the
settings are applied correctly. I just only have proto boards and
need someone having EVT boards to confirm the settings.
Original-Signed-off-by: Kenji Chen <kenji.chen@intel.com>
Original-Change-Id: Id1b5a52ff0b896f4531c4a6e68e70a2cea8c736a
Original-Reviewed-on: https://chromium-review.googlesource.com/221436
Original-Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: http://review.coreboot.org/8832
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/include/device/pci.h

@@ -33,6 +33,7 @@
 struct pci_operations {
 	/* set the Subsystem IDs for the PCI device */
 	void (*set_subsystem)(device_t dev, unsigned vendor, unsigned device);
+	void (*set_L1_ss_latency)(device_t dev, unsigned int off);
 };
 
 /* Common pci bus operations */