arm64: Implement bus_describe_intr() for nexus

Prompted by a compiler warning introduced by
e582d4a2b0 ("arm64: nexus code tidy-up").

Reviewed by:	mhorne, andrew
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D39749

sys/arm64/arm64/nexus.c

@@ -146,6 +146,7 @@ static device_method_t nexus_methods[] = {
 	DEVMETHOD(bus_bind_intr,	nexus_bind_intr),
 #endif
 	DEVMETHOD(bus_config_intr,	nexus_config_intr),
+	DEVMETHOD(bus_describe_intr,	nexus_describe_intr),
 	DEVMETHOD(bus_setup_intr,	nexus_setup_intr),
 	DEVMETHOD(bus_teardown_intr,	nexus_teardown_intr),
 	DEVMETHOD(bus_get_bus_tag,	nexus_get_bus_tag),
@@ -348,6 +349,14 @@ nexus_teardown_intr(device_t dev, device_t child, struct resource *r, void *ih)
 	return (intr_teardown_irq(child, r, ih));
 }
 
+static int
+nexus_describe_intr(device_t dev, device_t child, struct resource *irq,
+    void *cookie, const char *descr)
+{
+
+	return (intr_describe_irq(child, irq, cookie, descr));
+}
+
 #ifdef SMP
 static int
 nexus_bind_intr(device_t dev, device_t child, struct resource *irq, int cpu)