dt-bindings: firmware: qcom-scm: Document msm8953 bindings

SCM driver on MSM8953 requires 3 clocks.

Signed-off-by: Vladimir Lypak <junak.pub@gmail.com>
Signed-off-by: Sireesh Kodali <sireeshkodali@protonmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20210810164347.45578-4-sireeshkodali@protonmail.com