coresight: etm4x: docs: Update ABI doc for new sysfs etm4 attributes

This updates the ABI document to reflect recent additions to the ETM4.X
driver sysfs interface.

Signed-off-by: Mike Leach <mike.leach@linaro.org>
[Updated Date and KernelVersion fields]
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>