mailbox: Add snapshot of apcs-ipc driver

This snapshot is taken as of msm-4.14 'commit <f63747b877cd> ("Merge
"UPSTREAM commit 'eec334785ba1a5112db705ddcf5577e4bccc6009' 09/30"")'.

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

Change-Id: I879a90df22206f5689fe047fc2c6e8756bb81f53
Signed-off-by: Chris Lew <clew@codeaurora.org>