dt-bindings: mailbox: qcom: 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.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>