soc: imx: gpc: Switch to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management.
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

drivers/soc/imx/gpc.c

+// SPDX-License-Identifier: GPL-2.0+
 /*
  * Copyright 2015-2017 Pengutronix, Lucas Stach <kernel@pengutronix.de>
  * Copyright 2011-2013 Freescale Semiconductor, Inc.
- *
- * The code contained herein is licensed under the GNU General Public
- * License. You may obtain a copy of the GNU General Public License
- * Version 2 or later at the following locations:
- *
- * http://www.opensource.org/licenses/gpl-license.html
- * http://www.gnu.org/copyleft/gpl.html
  */
 
 #include <linux/clk.h>