dt-bindings: mailbox: Introduce MSM_QMP bindings

The msm_qmp driver is used to send packets to the aop. The messages
are sent over msgram owned by aop. This is a lightweight protocol that
only supports one packet in flight in any direction.

Change-Id: I411f18081d04a2cbb78a2091b295201ab6569258
Signed-off-by: Chris Lew <clew@codeaurora.org>