pwm: 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: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-imx.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * simple driver for PWM (Pulse Width Modulator) controller
  *
- * 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.
- *
  * Derived from pxa PWM driver by eric miao <eric.miao@marvell.com>
  */