doc: dt-bindings: mem-offline: Add doc for QMP mailbox communication

Add documentation for QMP mailbox communication
support in mem offline driver which enables support to
request that memory be physically onlined and offlined
when it is logically onlined and offlined.

Change-Id: I3dce774de3d0127b254a48520e1bb08272805f75
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>