nds32: Add document for NDS32 PMU.

The document for how to add NDS32 PMU
in devicetree.

Signed-off-by: Nickhu <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Acked-by: Greentime Hu <[e-mail]>
Signed-off-by: Greentime Hu <[e-mail]>

Documentation/devicetree/bindings/perf/nds32v3-pmu.txt

···
+* NDS32 Performance Monitor Units
+
+NDS32 core have a PMU for counting cpu and cache events like cache misses.
+The NDS32 PMU representation in the device tree should be done as under:
+
+Required properties:
+
+- compatible :
+	"andestech,nds32v3-pmu"
+
+- interrupts : The interrupt number for NDS32 PMU is 13.
+
+Example:
+pmu{
+	compatible = "andestech,nds32v3-pmu";
+	interrupts = <13>;
+}