dt-bindings: mailbox: qcom: Add SC8180X APCS compatible

Add compatible for the Qualcomm SC8180x APCS block to the Qualcomm APCS
binding.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>