nds32: Add document for NDS32 PMU.

The document for how to add NDS32 PMU
in devicetree.
Signed-off-by: NNickhu <nickhu@andestech.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Acked-by: NGreentime Hu <greentime@andestech.com>
Signed-off-by: NGreentime Hu <greentime@andestech.com>

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>;
+}