gpio: xtensa: Cut down on boilerplate

Just use the SPDX header for the license.
Acked-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-xtensa.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Copyright (C) 2013 TangoTec Ltd.
  * Author: Baruch Siach <baruch@tkos.co.il>
  *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2 as
- * published by the Free Software Foundation.
- *
  * Driver for the Xtensa LX4 GPIO32 Option
  *
  * Documentation: Xtensa LX4 Microprocessor Data Book, Section 2.22