dt-bindings: nds32 CPU Bindings

This patch adds nds32 CPU binding documents.
Signed-off-by: NVincent Chen <vincentc@andestech.com>
Signed-off-by: NRick Chen <rick@andestech.com>
Signed-off-by: NZong Li <zong@andestech.com>
Signed-off-by: NGreentime Hu <greentime@andestech.com>
Reviewed-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/nds32/cpus.txt

+* Andestech Processor Binding
+
+This binding specifies what properties must be available in the device tree
+representation of a Andestech Processor Core, which is the root node in the
+tree.
+
+Required properties:
+
+	- compatible:
+		Usage: required
+		Value type: <string>
+		Definition: Should be "andestech,<core_name>", "andestech,nds32v3" as fallback.
+		Must contain "andestech,nds32v3" as the most generic value, in addition to
+		one of the following identifiers for a particular CPU core:
+		"andestech,n13"
+		"andestech,n15"
+		"andestech,d15"
+		"andestech,n10"
+		"andestech,d10"
+	- device_type
+		Usage: required
+		Value type: <string>
+		Definition: must be "cpu"
+	- reg: Contains CPU index.
+	- clock-frequency: Contains the clock frequency for CPU, in Hz.
+
+* Examples
+
+/ {
+	cpus {
+		cpu@0 {
+			device_type = "cpu";
+			compatible = "andestech,n13", "andestech,nds32v3";
+			reg = <0x0>;
+			clock-frequency = <60000000>
+		};
+	};
+};