dt-bindings: gpu: mali-bifrost: Add Rockchip PX30

Define a compatible string for the Mali Bifrost GPU found in
Rockchip's PX30 SoCs.
Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Link: https://lore.kernel.org/r/20191208145508.3124-1-heiko@sntech.de

Documentation/devicetree/bindings/gpu/arm,mali-bifrost.yaml

@@ -18,6 +18,7 @@ properties:
       - enum:
           - amlogic,meson-g12a-mali
           - realtek,rtd1619-mali
+          - rockchip,px30-mali
       - const: arm,mali-bifrost # Mali Bifrost GPU model/revision is fully discoverable
 
   reg: