dt-bindings: add bindings for QMP debug interface

QMP debugfs client:

-----------------

QTI Messaging Protocol(QMP) debugfs client is an interface for clients to

send data to the Always on processor using QMP.

Required properties :

- compatible : must be "qcom,debugfs-qmp-client"

- mboxes : list of QMP mailbox phandle and channel identifier tuples.

- mbox-names : names of the listed mboxes

Example :

	qcom,qmp-client {

		compatible = "qcom,debugfs-qmp-client";

		mboxes = <&qmp_aop 0>;

		mbox-names = "aop";

	};