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-acpi.c

@@ -1,13 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * ACPI helpers for GPIO API
  *
  * Copyright (C) 2012, Intel Corporation
  * Authors: Mathias Nyman <mathias.nyman@linux.intel.com>
  *          Mika Westerberg <mika.westerberg@linux.intel.com>
- *
- * 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.
  */
 
 #include <linux/errno.h>