PCI: document the pcie_aspm kernel parameter

It can be handy so make sure people know about it.
Signed-off-by: NChuck Ebbert <cebbert@redhat.com>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>

Documentation/kernel-parameters.txt

@@ -1678,6 +1678,12 @@ and is between 256 and 4096 characters. It is defined in the file
 				reserved for the CardBus bridge's memory
 				window. The default value is 64 megabytes.
 
+	pcie_aspm=	[PCIE] Forcibly enable or disable PCIe Active State Power
+			Management.
+		off	Disable ASPM.
+		force	Enable ASPM even on devices that claim not to support it.
+			WARNING: Forcing ASPM on may cause system lockups.
+
 	pcmv=		[HW,PCMCIA] BadgePAD 4
 
 	pd.		[PARIDE]