dt-bindings: gpio-mmio: Add MMIO for IXP4xx expansion bus

This expansion is a simple MMIO-mapped GPIO device but the bus has a
number of additional properties that we need to bring in using
a reference to the bus child node schema.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250822-ixp4xx-eb-mmio-gpio-v2-2-bd2edd4a9c74@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Documentation/devicetree/bindings/gpio/gpio-mmio.yaml

@@ -22,6 +22,7 @@ properties:
       - brcm,bcm6345-gpio
       - ni,169445-nand-gpio
       - wd,mbl-gpio # Western Digital MyBook Live memory-mapped GPIO controller
+      - intel,ixp4xx-expansion-bus-mmio-gpio
 
   big-endian: true
 
@@ -89,6 +90,14 @@ properties:
     description:
       If this property is present, the controller cannot drive the GPIO lines.
 
+if:
+  properties:
+    compatible:
+      contains:
+        const: intel,ixp4xx-expansion-bus-mmio-gpio
+then:
+  $ref: /schemas/memory-controllers/intel,ixp4xx-expansion-peripheral-props.yaml#
+
 patternProperties:
   "^.+-hog(-[0-9]+)?$":
     type: object
@@ -102,7 +111,7 @@ required:
   - '#gpio-cells'
   - gpio-controller
 
-additionalProperties: false
+unevaluatedProperties: false
 
 examples:
   - |
@@ -132,3 +141,22 @@ examples:
       gpio-controller;
       #gpio-cells = <2>;
     };
+
+    bus@c4000000 {
+        compatible = "intel,ixp42x-expansion-bus-controller", "syscon";
+        reg = <0xc4000000 0x30>;
+        native-endian;
+        #address-cells = <2>;
+        #size-cells = <1>;
+        ranges = <0 0x0 0x50000000 0x01000000>;
+        dma-ranges = <0 0x0 0x50000000 0x01000000>;
+        gpio@1,0 {
+            compatible = "intel,ixp4xx-expansion-bus-mmio-gpio";
+            gpio-controller;
+            #gpio-cells = <2>;
+            big-endian;
+            reg = <1 0x00000000 0x2>;
+            reg-names = "dat";
+            intel,ixp4xx-eb-write-enable = <1>;
+        };
+    };