coresight: Update documentation for perf usage

Add notes on using perf to collect and analyze CoreSight trace

Signed-off-by: Robert Walker <robert.walker@arm.com>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: coresight@lists.linaro.org
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/1518607481-4059-4-git-send-email-robert.walker@arm.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>