PCI/PM: Add ABI document for sysfs file d3cold_allowed

This patch adds ABI document for the following sysfs file:

/sys/bus/pci/devices/.../d3cold_allowed
Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>

Documentation/ABI/testing/sysfs-bus-pci

@@ -210,3 +210,15 @@ Users:
 		firmware assigned instance number of the PCI
 		device that can help in understanding the firmware
 		intended order of the PCI device.
+
+What:		/sys/bus/pci/devices/.../d3cold_allowed
+Date:		July 2012
+Contact:	Huang Ying <ying.huang@intel.com>
+Description:
+		d3cold_allowed is bit to control whether the corresponding PCI
+		device can be put into D3Cold state.  If it is cleared, the
+		device will never be put into D3Cold state.  If it is set, the
+		device may be put into D3Cold state if other requirements are
+		satisfied too.  Reading this attribute will show the current
+		value of d3cold_allowed bit.  Writing this attribute will set
+		the value of d3cold_allowed bit.