extres/syscon_generic: Make device quiet if not in boot verbose

On some boards there is a lot of of syscon node that are unused as
more specific drivers is probed before, no need to flood the console
for the mostly-unused generic ones.

MFC after:	1 week

sys/dev/extres/syscon/syscon_generic.c

@@ -142,6 +142,9 @@ syscon_generic_probe(device_t dev)
 		return (ENXIO);
 
 	device_set_desc(dev, "syscon");
+	if (!bootverbose)
+		device_quiet(dev);
+
 	return (BUS_PROBE_GENERIC);
 }