dt-bindings: iio: adc: Add TI TWL603X GPADC

Document TI TWL603X GPADC devicetree bindings.
A driver is already there, the compatibles are used, but not documented.
Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230820123229.530371-1-andreas@kemnade.infoSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/ti,twl6030-gpadc.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/adc/ti,twl6030-gpadc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: GPADC subsystem in the TWL6030 power module
+
+maintainers:
+  - Andreas Kemnade <andreas@kemnade.info>
+
+description:
+  The GPADC subsystem in the TWL603X consists of a 10-bit ADC
+  combined with a 15-input analog multiplexer in the TWL6030 resp. a
+  19-input analog muliplexer in the TWL6032.
+
+properties:
+  compatible:
+    enum:
+      - ti,twl6030-gpadc
+      - ti,twl6032-gpadc
+
+  interrupts:
+    maxItems: 1
+
+  "#io-channel-cells":
+    const: 1
+
+required:
+  - compatible
+  - interrupts
+  - "#io-channel-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    gpadc {
+        compatible = "ti,twl6030-gpadc";
+        interrupts = <3>;
+        #io-channel-cells = <1>;
+    };
+...