dt-bindings: csky: Add csky PMU bindings

This patch adds the documentation to describe that how to add pmu node in
dts.
Signed-off-by: Mao Han <han_mao@c-sky.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Cc: Rob Herring <robh+dt@kernel.org>

Documentation/devicetree/bindings/csky/pmu.txt

+===============================
+C-SKY Performance Monitor Units
+===============================
+
+C-SKY Performance Monitor is designed for ck807/ck810/ck860 SMP soc and
+it could count cpu's events for helping analysis performance issues.
+
+============================
+PMU node bindings definition
+============================
+
+	Description: Describes PMU
+
+	PROPERTIES
+
+	- compatible
+		Usage: required
+		Value type: <string>
+		Definition: must be "csky,csky-pmu"
+	- interrupts
+		Usage: required
+		Value type: <u32 IRQ_TYPE_XXX>
+		Definition: must be pmu irq num defined by soc
+	- count-width
+		Usage: optional
+		Value type: <u32>
+		Definition: the width of pmu counter
+
+Examples:
+---------
+#include <dt-bindings/interrupt-controller/irq.h>
+
+	pmu: performace-monitor {
+		compatible = "csky,csky-pmu";
+		interrupts = <23 IRQ_TYPE_EDGE_RISING>;
+		interrupt-parent = <&intc>;
+		count-width = <48>;
+        };