Add a detach for pci bridge and pci bus drivers. This allows one to

theoretically unload pci bridges or pci drivers.  It will also allow
detach to work if one needed to detach a subtree.

This is inspired by looking at the p4 commits from bms to his 5.4
tree, but I didn't look at the final results.

sys/dev/pci/pci.c

@@ -96,6 +96,7 @@ static device_method_t pci_methods[] = {
 	/* Device interface */
 	DEVMETHOD(device_probe,		pci_probe),
 	DEVMETHOD(device_attach,	pci_attach),
+	DEVMETHOD(device_detach,	bus_generic_detach),
 	DEVMETHOD(device_shutdown,	bus_generic_shutdown),
 	DEVMETHOD(device_suspend,	pci_suspend),
 	DEVMETHOD(device_resume,	pci_resume),

sys/dev/pci/pci_pci.c

@@ -58,6 +58,7 @@ static device_method_t pcib_methods[] = {
     /* Device interface */
     DEVMETHOD(device_probe,		pcib_probe),
     DEVMETHOD(device_attach,		pcib_attach),
+    DEVMETHOD(device_detach,		bus_generic_detach),
     DEVMETHOD(device_shutdown,		bus_generic_shutdown),
     DEVMETHOD(device_suspend,		bus_generic_suspend),
     DEVMETHOD(device_resume,		bus_generic_resume),