Adds intel_gpu_trace_pmu + intel_oacounter_top_pmu  tools

This adds some minimal utilities for testing the i915_oa perf pmu
driver. intel_gpu_trace_pmu is able to log all of the data received via
the mmaped circular samples buffer into a .json file that can be
visualised via the chrome://tracing ui. intel_oacounter_top_pmu is
similar to intel_gpu_top in that it provides a system wide performance
analysis view, but just for OA counters,