coresight: etm4x: docs: Add documentation for 'ts_source' sysfs interface

Sync sysfs documentation pages to include the new ts_source (timestamp
source) interface.

Signed-off-by: German Gomez <german.gomez@arm.com>
Signed-off-by: James Clark <james.clark@arm.com>
Link: https://lore.kernel.org/r/20220823160650.455823-3-james.clark@arm.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>