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>

Documentation/devicetree/bindings/mailbox/qcom,apcs-kpss-global.yaml

@@ -166,6 +166,7 @@ examples:
         reg = <0x9820000 0x1000>;
 
         #mbox-cells = <1>;
+        #clock-cells = <0>;
     };
 
     rpm-glink {