dev/ofw: Register the ofw_cpu xref

This allows it to be referenced by other devices.

Reviewed by:	jhibbits
Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D53329

sys/dev/ofw/ofw_cpu.c

@@ -332,6 +332,7 @@ ofw_cpu_attach(device_t dev)
 		device_printf(dev, "Nominal frequency %dMhz\n",
 		    sc->sc_nominal_mhz);
 
+	OF_device_register_xref(OF_xref_from_node(node), dev);
 	bus_identify_children(dev);
 	bus_attach_children(dev);
 	return (0);