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>

Documentation/devicetree/bindings/iio/adc/adi,ad7768-1.yaml

@@ -70,6 +70,14 @@ properties:
   "#io-channel-cells":
     const: 1
 
+  gpio-controller: true
+
+  "#gpio-cells":
+    const: 2
+    description: |
+      The first cell is for the GPIO number: 0 to 3.
+      The second cell takes standard GPIO flags.
+
 required:
   - compatible
   - reg
@@ -118,6 +126,8 @@ examples:
             spi-max-frequency = <2000000>;
             spi-cpol;
             spi-cpha;
+            gpio-controller;
+            #gpio-cells = <2>;
             vref-supply = <&adc_vref>;
             interrupts = <25 IRQ_TYPE_EDGE_RISING>;
             interrupt-parent = <&gpio>;