coresight: etm4x: docs: Adds detailed document for programming etm4x.

Add in detailed programmers reference for users wanting to program the
CoreSight ETM 4.x driver using sysfs.

Signed-off-by: Mike Leach <mike.leach@linaro.org>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>