dt-bindings: interrupt-controller: C-SKY SMP intc

Dt-bindings doc about C-SKY Multi-processors interrupt controller.

Changelog:
 - Should be: '#interrupt-cells' not 'interrupt-cells'
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Reviewed-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/interrupt-controller/csky,mpintc.txt

+===========================================
+C-SKY Multi-processors Interrupt Controller
+===========================================
+
+C-SKY Multi-processors Interrupt Controller is designed for ck807/ck810/ck860
+SMP soc, and it also could be used in non-SMP system.
+
+Interrupt number definition:
+
+  0-15  : software irq, and we use 15 as our IPI_IRQ.
+ 16-31  : private  irq, and we use 16 as the co-processor timer.
+ 31-1024: common irq for soc ip.
+
+=============================
+intc node bindings definition
+=============================
+
+	Description: Describes SMP interrupt controller
+
+	PROPERTIES
+
+	- compatible
+		Usage: required
+		Value type: <string>
+		Definition: must be "csky,mpintc"
+	- #interrupt-cells
+		Usage: required
+		Value type: <u32>
+		Definition: must be <1>
+	- interrupt-controller:
+		Usage: required
+
+Examples:
+---------
+
+	intc: interrupt-controller {
+		compatible = "csky,mpintc";
+		#interrupt-cells = <1>;
+		interrupt-controller;
+	};