ARC: Add documentation on DT binding for ARC700 PMU

* 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";

};