dt-bindings: gpu: mali-utgard: Add Rockchip RK3128 compatible

Rockchip RK312x SoC family has a Mali400 MP2.
Add a compatible for it.

Signed-off-by: Alex Bee <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Heiko Stuebner <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

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

···
               - allwinner,sun50i-a64-mali
               - rockchip,rk3036-mali
               - rockchip,rk3066-mali
+              - rockchip,rk3128-mali
               - rockchip,rk3188-mali
               - rockchip,rk3228-mali
               - samsung,exynos4210-mali