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/common/spl/Makefile b/common/spl/Makefile
index a74563c..e8144f7 100644
--- a/common/spl/Makefile
+++ b/common/spl/Makefile
@@ -3,8 +3,7 @@
 # Texas Instruments Incorporated - http://www.ti.com/
 # Aneesh V <aneesh@ti.com>
 #
-# This file is released under the terms of GPL v2 and any later version.
-# See the file COPYING in the root directory of the source tree for details.
+# SPDX-License-Identifier:	GPL-2.0+
 #
 # Based on common/Makefile.
 #