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 <knaerzche@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20231202125144.66052-4-knaerzche@gmail.com