dt-bindings: Document devicetree binding for ARM SPE

* ARMv8.2 Statistical Profiling Extension (SPE) Performance Monitor Units (PMU)

ARMv8.2 introduces the optional Statistical Profiling Extension for collecting

performance sample data using an in-memory trace buffer.

** SPE Required properties:

- compatible : should be one of:

	       "arm,statistical-profiling-extension-v1"

- interrupts : Exactly 1 PPI must be listed. For heterogeneous systems where

               SPE is only supported on a subset of the CPUs, please consult

	       the arm,gic-v3 binding for details on describing a PPI partition.

** Example:

spe-pmu {

        compatible = "arm,statistical-profiling-extension-v1";

        interrupts = <GIC_PPI 05 IRQ_TYPE_LEVEL_HIGH &part1>;

};