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>