arm64: dts: qcom: msm8916: Probe the APCS mailbox driver

The APCS block was exposed until now as a syscon, but now we have a
proper driver for this block. Add the compatible string of the new
driver to probe and register the mailbox functionality.

Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>