perf test coresight: Add relevant documentation about ARM64 CoreSight testing

perf test coresight: Add relevant documentation about ARM64 CoreSight testing

Add/improve documentation helping people get started with CoreSight and
perf as well as describe the testing and how it works.

Reviewed-by: James Clark <james.clark@arm.com>
Signed-off-by: Carsten Haitzler <carsten.haitzler@arm.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Suzuki Poulouse <suzuki.poulose@arm.com>
Cc: coresight@lists.linaro.org
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20220909152803.2317006-14-carsten.haitzler@foss.arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>