dt-bindings: gpu: mali-utgard: add rockchip,rk3328-mali compatible

The rk3328 quad-core Cortex A53 uses a Mali-450MP2 with 2 PPs, so
add a compatible for it.
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/gpu/arm,mali-utgard.txt

@@ -17,6 +17,7 @@ Required properties:
       + rockchip,rk3066-mali
       + rockchip,rk3188-mali
       + rockchip,rk3228-mali
+      + rockchip,rk3328-mali
       + stericsson,db8500-mali
 
   - reg: Physical base address and length of the GPU registers