dt-bindings: mailbox: qcom: Add MSM8939 APCS compatible

Add compatible for the Qualcomm MSM8939 APCS block to the Qualcomm APCS
bindings.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>