dt-bindings: drm/msm/gpu: Add cooling device support

Add cooling device support to gpu. A cooling device is bound to a
thermal zone to allow thermal mitigation.

Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>