iio: adc: sun4i-gpadc-iio convert to SPDX license tags

Updates license to use SPDX-License-Identifier.
Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/sun4i-gpadc-iio.c

+// SPDX-License-Identifier: GPL-2.0
 /* ADC driver for sunxi platforms' (A10, A13 and A31) GPADC
  *
  * Copyright (c) 2016 Quentin Schulz <quentin.schulz@free-electrons.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.
- *
  * The Allwinner SoCs all have an ADC that can also act as a touchscreen
  * controller and a thermal sensor.
  * The thermal sensor works only when the ADC acts as a touchscreen controller