dt-bindings: interrupt-controller: mediatek,mt6577-sysirq: Drop invalid JSON pointer

The $ref "arm,gic.yaml#/properties/#interrupt-cells" doesn't work when
used with another json-schema implementation (rust). The issue is it not
a valid URI due to the 2nd "#" character in the JSON pointer. Though a
"#" is supposed to be valid within a JSON pointer. A possible solution
is to encode the "#" as "%23" instead, but the dtschema tools don't
handle that.

Just avoid all these issues as the $ref only resolves to a "const: 3"
schema and specify it directly.

Link: https://patch.msgid.link/20260813145453.1919998-1-robh@kernel.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

Documentation/devicetree/bindings/interrupt-controller/mediatek,mt6577-sysirq.yaml

@@ -49,7 +49,8 @@ properties:
   interrupt-controller: true
 
   "#interrupt-cells":
-    $ref: arm,gic.yaml#/properties/#interrupt-cells
+    const: 3
+    description: The cells have the same encoding as defined in arm,gic.yaml.
 
 required:
   - reg