coresight: tmc-etr: Introduce the notion of reference counting to ETR devices

This patch adds reference counting to struct etr_buf so that, in CPU-wide
trace scenarios, shared buffers can be disposed of when no longer used.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Tested-by: Leo Yan <leo.yan@linaro.org>
Tested-by: Robert Walker <robert.walker@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>