dt-bindings: iio: adc: ad7768-1: Document GPIO controller

dt-bindings: iio: adc: ad7768-1: Document GPIO controller

The AD7768-1 ADC exports four bidirectional GPIOs accessible
via register map.

Document GPIO properties necessary to enable GPIO controller for this
device.

Acked-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Link: https://patch.msgid.link/2ac34fc1e0b02886073ae0bb196c7e8d4d442c3f.1749569957.git.Jonathan.Santos@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>