coresight: Add section for integration with the perf tools

Adding a section that document how to use the Coresight framework and
drivers from the perf tools.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>