mtd: rawnand: vf610: Add an SPDX tag to replace the license text

Replace the license text by an SPDX tag.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>