gpio: Use SPDX header for core library

Use the SPDX headers and cut down on boilerplate to indicate the
license in the core gpiolib implementation.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib-of.c

@@ -1,14 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * OF helpers for the GPIO API
  *
  * Copyright (c) 2007-2008  MontaVista Software, Inc.
  *
  * Author: Anton Vorontsov <avorontsov@ru.mvista.com>
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation; either version 2 of the License, or
- * (at your option) any later version.
  */
 
 #include <linux/device.h>