dt-bindings: clock: document SM8550 DISPCC clock controller

Document device tree bindings for display clock controller for
Qualcomm SM8550 SoC.

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230103-topic-sm8550-upstream-dispcc-v3-1-8a03d348c572@linaro.org