ARC: Add documentation on DT binding for ARC700 PMU

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

Documentation/devicetree/bindings/arc/pmu.txt

+* ARC Performance Monitor Unit
+
+The ARC 700 can be configured with a pipeline performance monitor for counting
+CPU and cache events like cache misses and hits.
+
+Note that:
+ * ARC 700 refers to a family of ARC processor cores;
+   - There is only one type of PMU available for the whole family;
+   - The PMU may support different sets of events; supported events are probed
+     at boot time, as required by the reference manual.
+
+ * The ARC 700 PMU does not support interrupts; although HW events may be
+   counted, the HW events themselves cannot serve as a trigger for a sample.
+
+Required properties:
+
+- compatible : should contain
+	"snps,arc700-pmu"
+
+Example:
+
+pmu {
+        compatible = "snps,arc700-pmu";
+};