thermal: imx: 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: Eduardo Valentin <edubezval@gmail.com>

drivers/thermal/imx_thermal.c

-/*
- * Copyright 2013 Freescale Semiconductor, Inc.
- *
- * 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.
- *
- */
+// SPDX-License-Identifier: GPL-2.0
+//
+// Copyright 2013 Freescale Semiconductor, Inc.
 
 #include <linux/clk.h>
 #include <linux/cpufreq.h>