soc: qcom: Add device tree binding for Shared Memory Device

Qualcomm Shared Memory Driver (SMD) binding

This binding describes the Qualcomm Shared Memory Driver, a fifo based

communication channel for sending data between the various subsystems in

Qualcomm platforms.

- compatible:

	Usage: required

	Value type: <stringlist>

	Definition: must be "qcom,smd"

= EDGES

Each subnode of the SMD node represents a remote subsystem or a remote

processor of some sort - or in SMD language an "edge". The name of the edges

are not important.

The edge is described by the following properties:

- interrupts:

	Usage: required

	Value type: <prop-encoded-array>

	Definition: should specify the IRQ used by the remote processor to

		    signal this processor about communication related updates

- qcom,ipc:

	Usage: required

	Value type: <prop-encoded-array>

	Definition: three entries specifying the outgoing ipc bit used for

		    signaling the remote processor:

		    - phandle to a syscon node representing the apcs registers

		    - u32 representing offset to the register within the syscon

		    - u32 representing the ipc bit within the register

- qcom,smd-edge:

	Usage: required

	Value type: <u32>

	Definition: the identifier of the remote processor in the smd channel

		    allocation table

= SMD DEVICES

In turn, subnodes of the "edges" represent devices tied to SMD channels on that

"edge". The names of the devices are not important. The properties of these

nodes are defined by the individual bindings for the SMD devices - but must

contain the following property:

- qcom,smd-channels:

	Usage: required

	Value type: <stringlist>

	Definition: a list of channels tied to this device, used for matching

		    the device to channels

= EXAMPLE

The following example represents a smd node, with one edge representing the

"rpm" subsystem. For the "rpm" subsystem we have a device tied to the

"rpm_request" channel.

	apcs: syscon@f9011000 {

		compatible = "syscon";

		reg = <0xf9011000 0x1000>;

	};

	smd {

		compatible = "qcom,smd";

		rpm {

			interrupts = <0 168 1>;

			qcom,ipc = <&apcs 8 0>;

			qcom,smd-edge = <15>;

			rpm_requests {

				compatible = "qcom,rpm-msm8974";

				qcom,smd-channels = "rpm_requests";

				...

			};

		};

	};