iio: adc: ad4170-4: Add GPIO controller support

iio: adc: ad4170-4: Add GPIO controller support

The AD4170-4 has four multifunctional pins that can be used as GPIOs. The
GPIO functionality can be accessed when the AD4170-4 chip is not busy
performing continuous data capture or handling any other register
read/write request. Also, the AD4170-4 does not provide any interrupt based
on GPIO pin states so AD4170-4 GPIOs can't be used as interrupt sources.

Implement gpio_chip callbacks to make AD4170-4 GPIO pins controllable
through the gpiochip interface.

Acked-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/e031189d4b7e20cf02dd13220ab1ddf4798760c2.1751895245.git.marcelo.schmitt@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>