dt-bindings: firmware: qcom-scm: Document msm8976 bindings

SCM driver on MSM8976 requires 3 clocks.

Signed-off-by: Adam Skladowski <a39.skl@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220307191726.10869-4-a39.skl@gmail.com