soc: qcom: Add device tree binding for GLINK RPM

Qualcomm RPM GLINK binding

This binding describes the Qualcomm RPM GLINK, a fifo based mechanism for

communication with the Resource Power Management system on various Qualcomm

platforms.

- compatible:

	Usage: required

	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

	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";

			...

		};

	};