dt-bindings: mailbox: qcom: add #clock-cells to msm8996 example

Add the #clock-cells property to the MSM8996 example, as the APCS block
is going to provide the `sys_apcs_aux' clock to the consumers.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>