mtd: spi-nor: Add an SPDX tag to spi-nor.{c,h}

Add SPDX tags to replace the license boiler-plate and fix the
MODULE_LICENSE() definition in spi-nor.c to match the license text
(GPL v2).

Interestingly, spi-nor.h and spi-nor.c do not use the same license
(GPL v2+ for spi-nor.h, GPL v2 for spi-nor.c).

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>

include/linux/mtd/spi-nor.h

@@ -1,10 +1,6 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * Copyright (C) 2014 Freescale Semiconductor, Inc.
- *
- * 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.
  */
 
 #ifndef __LINUX_MTD_SPI_NOR_H