pwm: mediatek: Update license and switch to SPDX tag

Add SPDX identifiers to pwm-mediatek.c. Update MODULE_LICENSE to
correctly reflect the GNU General Public License v2.0.
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Sam Shih <sam.shih@mediatek.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-mediatek.c

+// SPDX-License-Identifier: GPL-2.0
 /*
- * Mediatek Pulse Width Modulator driver
+ * MediaTek Pulse Width Modulator driver
  *
  * Copyright (C) 2015 John Crispin <blogic@openwrt.org>
  * Copyright (C) 2017 Zhi Mao <zhi.mao@mediatek.com>
  *
- * This file is licensed under the terms of the GNU General Public
- * License version 2. This program is licensed "as is" without any
- * warranty of any kind, whether express or implied.
  */
 
 #include <linux/err.h>
@@ -325,4 +323,4 @@ static struct platform_driver pwm_mediatek_driver = {
 module_platform_driver(pwm_mediatek_driver);
 
 MODULE_AUTHOR("John Crispin <blogic@openwrt.org>");
-MODULE_LICENSE("GPL");
+MODULE_LICENSE("GPL v2");