coresight: pmu: Adds return stack option to perf coresight pmu

Return stack is a programmable option on some ETM and PTM hardware.
Adds the option flags to enable this from the perf event command line.

Signed-off-by: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>