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>