dt-bindings: gpu: add the RK3399 mali for rockchip specifics

RK3399's GPU uses the quad-core Mali-T860, which is the new generation of
high-end graphics processors from ARM.

This patch added "rockchip,rk3399-mali" for dt-bindings, in order to
support IPA of gpu thermal in later.

Signed-off-by: Caesar Wang <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Heiko Stuebner <[e-mail]>

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

···
   * which must be preceded by one of the following vendor specifics:
     + "amlogic,meson-gxm-mali"
     + "rockchip,rk3288-mali"
+    + "rockchip,rk3399-mali"
 
 - reg : Physical base address of the device and length of the register area.