dt-bindings: soc: qcom: spm: Add MSM8909 CPU compatible

Document the "qcom,msm8909-saw2-v3.0-cpu" compatible for the CPU
Subsystem Power Manager (SPM) on the MSM8909 SoC. This is necessary
for CPU idle since this is a fairly old 32-bit SoC without support
for PSCI.

Signed-off-by: Stephan Gerhold <stephan.gerhold@kernkonzept.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220705143523.3390944-6-stephan.gerhold@kernkonzept.com