mainboard: Use DEVICE_NOOP macro over dummy symbol

Change-Id: I37c302c7a17645f55d568c1a5f7bb791319ddbb4
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7286
Tested-by: build bot (Jenkins)
Reviewed-by: Bruce Griffith <Bruce.Griffith@se-eng.com>
1 file changed