Add GPL-2.0+ SPDX-License-Identifier to source files

Signed-off-by: Wolfgang Denk <wd@denx.de>
[trini: Fixup common/cmd_io.c]
Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 870d8b4..12e8cc1 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -15,10 +15,7 @@
  *		Bjorn Ekwall. <bj0rn@blox.se>
  *              Pekka Riikonen <priikone@poseidon.pspt.fi>
  *
- *		This program is free software; you can redistribute it and/or
- *		modify it under the terms of the GNU General Public License
- *		as published by the Free Software Foundation; either version
- *		2 of the License, or (at your option) any later version.
+ * SPDX-License-Identifier:	GPL-2.0+
  *
  *		Moved to /usr/include/linux for NET3
  */