dt-bindings: mtd: Fix nand-controller.yaml license

Binding files should be dual licensed. This file was initially written
as a .txt file with no specific license, so was implicitely
GPLv2. Significant part of this file and its conversion into yaml were
written by Bootlin employees which agree to comply with the rules
regarding the dual licensing so let's fix the SPDX tag to reflect the
correct license by changing it from GPL to GPL + BSD-2-Clause.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/linux-mtd/20230712151042.433593-1-miquel.raynal@bootlin.com

Documentation/devicetree/bindings/mtd/nand-controller.yaml

-# SPDX-License-Identifier: GPL-2.0
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
 %YAML 1.2
 ---
 $id: http://devicetree.org/schemas/mtd/nand-controller.yaml#