dt-bindings: arm: qcom: Document SM8550 SoC and boards

Document the SM8550 SoC binding and the MTP board.

Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230106201047.337409-2-abel.vesa@linaro.org