drm/msm: Add support for GPU cooling

Register GPU as a devfreq cooling device so that it can be passively
cooled by the thermal framework.

Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
Tested-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>