mtd: maps: physmap: Add SPDX header

Add an SPDX header matching the MODULE_LICENSE("GPL") definition.
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>

drivers/mtd/maps/physmap.c

+// SPDX-License-Identifier: GPL-2.0+
 /*
  * Normal mappings of chips in physical memory
  *