dt-bindings: display/msm: document DPU on SM8550

Document the DPU hardware found on the Qualcomm SM8550 platform.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/517511/
Link: https://lore.kernel.org/r/20230103-topic-sm8550-upstream-mdss-dsi-v3-2-660c3bcb127f@linaro.org


[DB: removed interconnect header inclusion]
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>