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/drivers/mtd/spi/sf.c b/drivers/mtd/spi/sf.c
index ddbdda0..d5e175c 100644
--- a/drivers/mtd/spi/sf.c
+++ b/drivers/mtd/spi/sf.c
@@ -4,7 +4,7 @@
  * Copyright (C) 2008 Atmel Corporation
  * Copyright (C) 2010 Reinhard Meyer, EMK Elektronik
  *
- * Licensed under the GPL-2 or later.
+ * SPDX-License-Identifier:	GPL-2.0+
  */
 
 #include <common.h>