dt-bindings: mailbox: Add snapshot of apcs-ipc driver

This snapshot is taken as of msm-4.14 'commit <f63747b8> ("Merge
"UPSTREAM commit 'eec33478' 09/30"")'.

The qcom-apcs-ipc-mailbox driver maps the shared apps register to
trigger interrupts to remote subsystems.

Change-Id: I9fa3b288c00f1d72a6ab986087d86167211a69b3
Signed-off-by: Chris Lew <clew@codeaurora.org>