perf cs-etm: Allow no CoreSight sink to be specified on command line

Adjust the handling of the session sink selection to allow no sink to be
selected on the command line. This then forwards the sink selection to
the CoreSight infrastructure which will attempt to select a sink based
on the default sink select priorities.

Signed-off-by: Mike Leach <mike.leach@linaro.org>
Tested-by: Leo Yan <leo.yan@linaro.org>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Suzuki Poulouse <suzuki.poulose@arm.com>
Cc: coresight@lists.linaro.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>