sf: Add GPL-2.0+ SPDX-License-Identifier for missing ones

Added GPL-2.0+ SPDX-License-Identifier for missed sf
source files.

Signed-off-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
Signed-off-by: Bo Shen <voice.shen@atmel.com>
diff --git a/common/cmd_sf.c b/common/cmd_sf.c
index 3f60979..3994b06 100644
--- a/common/cmd_sf.c
+++ b/common/cmd_sf.c
@@ -2,7 +2,8 @@
  * Command for accessing SPI flash.
  *
  * Copyright (C) 2008 Atmel Corporation
- * Licensed under the GPL-2 or later.
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
  */
 
 #include <common.h>