dt-bindings: mailbox: qcom,apcs-kpss-global: Add clock-output-names

Add clock-output-names as optional so that SoCs such as the msm8939 which
have multiple a53 PLLs can latch the appropriate output name in
drivers/clk/qcom/apcs-msm8916.c.

Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>