target-unicore32: License future contributions under GPLv2+

This is to limit relicensing obstacles to the pending IBM investigation.
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Acked-by: NGuan Xuetao <gxt@mprc.pku.edu.cn>

target-unicore32/helper.c

@@ -4,6 +4,9 @@
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 as
  * published by the Free Software Foundation.
+ *
+ * Contributions from 2012-04-01 on are considered under GPL version 2,
+ * or (at your option) any later version.
  */
 
 #include "cpu.h"