dt-bindings: arm: msm: msm_qmp: Add mailbox binding

Add binding to use mailbox interface for triggering outgoing
interrupts. Use either the mailbox or the irq-reg binding.

Change-Id: I044fcd44e76f239125529506ec1354b5ec862e5a
Signed-off-by: Chris Lew <clew@codeaurora.org>