dt-bindings: mailbox: qcom,apcs-kpss-global: Add syscon const for relevant entries

msm8916, msm8939, msm8953, msm8994 and qcs404 already declare or should
declare syscon as they have drivers that use syscon inside of the apcs-kpss
block.

grep apcs arch/arm64/boot/dts/qcom/* | grep syscon

Add in the additional syscon in the documentation for the above mentioned
parts.

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