iio: adc: ti_am335x_adc: Replace license text with SPDX tag

Drop the text license and replace it with an equivalent SPDX license tag
identifier which also matches the MODULE_LICENSE("GPL").

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211015081506.933180-40-miquel.raynal@bootlin.com