[AGPGART] Add support for SiS 5591.

Untested, but from the looks of things (comparing with DragonFlyBSD's agp implementation)
this should work.  Strangely, we added the PCI ID for this a long time ago, back
in 2.0.34 (long before agpgart driver got merged) and then we never used it.
Signed-off-by: Dave Jones <davej@redhat.com>

drivers/char/agp/sis-agp.c

@@ -144,6 +144,10 @@ struct agp_bridge_driver sis_driver = {
 
 static struct agp_device_ids sis_agp_device_ids[] __devinitdata =
 {
+	{
+		.device_id	= PCI_DEVICE_ID_SI_5591_AGP,
+		.chipset_name	= "5591",
+	},
 	{
 		.device_id	= PCI_DEVICE_ID_SI_530,
 		.chipset_name	= "530",