dt-bindings: soc: qcom: Add devicetree binding for QCOM SPM

Add devicetree binding for Qualcomm Subsystem Power Manager (SPM).

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20210729155609.608159-3-angelogioacchino.delregno@somainline.org