dt-bindings: arm: bcm: raspberrypi,bcm2835-firmware: Add gpio child node

Unlike the other child nodes of the raspberrypi,bcm2835-firmware device,
the gpio child is documented in a legacy text-based binding in
gpio/raspberrypi,firmware-gpio.txt. This causes DT validation failures:

arch/arm64/boot/dts/broadcom/bcm2711-rpi-4-b.dtb: 'gpio' does not match any of the regexes: 'pinctrl-[0-9]+'
        from schema $id: http://devicetree.org/schemas/arm/bcm/raspberrypi,bcm2835-firmware.yaml#

Convert the binding to YAML and move it to
raspberrypi,bcm2835-firmware.yaml.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Stefan Wahren <wahrenst@gmx.net>
Tested-by: Ivan T. Ivanov <iivanov@suse.de>
Link: https://lore.kernel.org/r/20240326195807.15163-2-laurent.pinchart@ideasonboard.com
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>

Author: pinchartl

Documentation/devicetree/bindings/arm/bcm/raspberrypi,bcm2835-firmware.yaml

@@ -46,6 +46,30 @@ properties:
       - compatible
       - "#clock-cells"
 
+  gpio:
+    type: object
+    additionalProperties: false
+
+    properties:
+      compatible:
+        const: raspberrypi,firmware-gpio
+
+      gpio-controller: true
+
+      "#gpio-cells":
+        const: 2
+        description:
+          The first cell is the pin number, and the second cell is used to
+          specify the gpio polarity (GPIO_ACTIVE_HIGH or GPIO_ACTIVE_LOW).
+
+      gpio-line-names:
+        minItems: 8
+
+    required:
+      - compatible
+      - gpio-controller
+      - "#gpio-cells"
+
   reset:
     type: object
     additionalProperties: false
@@ -96,6 +120,12 @@ examples:
             #clock-cells = <1>;
         };
 
+        expgpio: gpio {
+            compatible = "raspberrypi,firmware-gpio";
+            gpio-controller;
+            #gpio-cells = <2>;
+        };
+
         reset: reset {
             compatible = "raspberrypi,firmware-reset";
             #reset-cells = <1>;