dt-bindings: soc: qcom: spm: Document qcom,msm8916-saw2-v3.0-cpu

Document the qcom,msm8916-saw2-v3.0-cpu compatible that is needed
for cpuidle for MSM8916 on some devices with signed firmware which
is only capable of booting ARM32 kernels without PSCI.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20211004204955.21077-10-stephan@gerhold.net