dt-bindings: drm/msm/gpu: Document gpu opp table

Update documentation to list the gpu opp table bindings including the
newly added "opp-peak-kBps" needed for GPU-DDR bandwidth scaling.

Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>