dt-bindings: firmware: document Qualcomm SM8550 SCM

Document the compatible for Qualcomm SM8550 SCM.

Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
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/20230207-topic-sm8550-upstream-scm-bindings-v2-1-ca12bd33fa1c@linaro.org