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 <wxt@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>