dt-bindings: arm: qcom: Add missing schema for MSM8974

The SoC/board bindings for MSM8974 are undocumented. Add the missing
bindings to the schema.

Cc: Andy Gross <agross@kernel.org>
Cc: David Brown <david.brown@linaro.org>
Cc: linux-arm-msm@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>