iio: adc: ti-ads7950: add GPIO support

The ADS79XX has GPIO pins that can be used. Add support for the GPIO
pins using the GPIO chip framework.

Signed-off-by: Justin Chen <justinpopo6@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>