Qualcomm GLINK edge binding

This binding describes a Qualcomm GLINK edge, a fifo based mechanism for
communication between subsystem-pairs on various Qualcomm platforms. Two types
of edges can be described by the binding; the GLINK RPM edge and a SMEM based
edge.

- compatible:
	Usage: required for glink-rpm
	Value type: <stringlist>
	Definition: must be "qcom,glink-rpm"

- 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 events

- qcom,rpm-msg-ram:
	Usage: required for glink-rpm
	Value type: <prop-encoded-array>
	Definition: handle to RPM message memory resource

- mboxes:
	Usage: required
	Value type: <prop-encoded-array>
	Definition: reference to the "rpm_hlos" mailbox in APCS, as described
		    in mailbox/mailbox.txt

= GLINK DEVICES
Each subnode of the GLINK node represent function tied to a virtual
communication channel. The name of the nodes are not important. The properties
of these nodes are defined by the individual bindings for the specific function
- but must contain the following property:

- qcom,glink-channels:
	Usage: required
	Value type: <stringlist>
	Definition: a list of channels tied to this function, used for matching
		    the function to a set of virtual channels

= EXAMPLE
The following example represents the GLINK RPM node on a MSM8996 device, with
the function for the "rpm_request" channel defined, which is used for
regualtors and root clocks.

	apcs_glb: mailbox@9820000 {
		compatible = "qcom,msm8996-apcs-hmss-global";
		reg = <0x9820000 0x1000>;

		#mbox-cells = <1>;
	};

	rpm_msg_ram: memory@68000 {
		compatible = "qcom,rpm-msg-ram";
		reg = <0x68000 0x6000>;
	};

	rpm-glink {
		compatible = "qcom,glink-rpm";

		interrupts = <GIC_SPI 168 IRQ_TYPE_EDGE_RISING>;

		qcom,rpm-msg-ram = <&rpm_msg_ram>;

		mboxes = <&apcs_glb 0>;

		rpm-requests {
			compatible = "qcom,rpm-msm8996";
			qcom,glink-channels = "rpm_requests";

			...
		};
	};