dt-bindings: mailbox: qcom: Add SM4250 APCS compatible

Add compatible for the Qualcomm SM4250 APCS block.

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>