dt-bindings: firmware: document Qualcomm SM6375 SCM

Document the compatible for Qualcomm SM6375 SCM.

SCM consumes a single clock (core / RPM_SMD_CE1_CLK), though it does
not matter whether Linux enables it, as one of the billion levels of
firmware ensures it's on anyway. Still, mark it as used for the sake
of correctness.

Signed-off-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221015115535.150037-1-konrad.dybcio@somainline.org