dt-bindings: firmware: scm: add sdm670 compatible

The Snapdragon 670 uses SCM as for PSCI power management. Document the
appropriate compatible string for it.

Signed-off-by: Richard Acayan <mailingradian@gmail.com>

Documentation/devicetree/bindings/firmware/qcom,scm.yaml

@@ -41,6 +41,7 @@ properties:
           - qcom,scm-sc7180
           - qcom,scm-sc7280
           - qcom,scm-sc8280xp
+          - qcom,scm-sdm670
           - qcom,scm-sdm845
           - qcom,scm-sdx55
           - qcom,scm-sdx65