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