soc: qcom: Add device tree binding for GLINK RPM

Add device tree binding documentation for the Qualcomm GLINK RPM, used
for communication with the Resource Power Management subsystem in
various Qualcomm SoCs.
Acked-by: Andy Gross <andy.gross@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

Documentation/devicetree/bindings/soc/qcom/qcom,glink.txt

+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";
+
+			...
+		};
+	};