dt-bindings: mailbox: qcom: set correct #clock-cells

IPQ6018 and IPQ8074 require #clock-cells to be set to 1 as their APSS
clock driver provides multiple clock outputs.

So allow setting 1 as #clock-cells and check that its set to 1 for IPQ6018
and IPQ8074, check others for 0 as its currently.

Signed-off-by: Robert Marko <robimarko@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>