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.comSigned-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>

Documentation/ABI/testing/sysfs-bus-coresight-devices-etm4x

@@ -516,3 +516,11 @@ Contact:	Mathieu Poirier <mathieu.poirier@linaro.org>
 Description:	(Read) Returns the number of special conditional P1 right-hand keys
 		that the trace unit can use (0x194).  The value is taken
 		directly from the HW.
+
+What:		/sys/bus/coresight/devices/etm<N>/ts_source
+Date:		October 2022
+KernelVersion:	6.1
+Contact:	Mathieu Poirier <mathieu.poirier@linaro.org> or Suzuki K Poulose <suzuki.poulose@arm.com>
+Description:	(Read) When FEAT_TRF is implemented, value of TRFCR_ELx.TS used for
+		trace session. Otherwise -1 indicates an unknown time source. Check
+		trcidr0.tssize to see if a global timestamp is available.

Documentation/trace/coresight/coresight-etm4x-reference.rst

@@ -71,6 +71,20 @@ the ‘TRC’ prefix.
 
 ----
 
+:File:            ``ts_source`` (ro)
+:Trace Registers: None.
+:Notes:
+    When FEAT_TRF is implemented, value of TRFCR_ELx.TS used for trace session. Otherwise -1
+    indicates an unknown time source. Check trcidr0.tssize to see if a global timestamp is
+    available.
+
+:Example:
+    ``$> cat ts_source``
+
+    ``$> 1``
+
+----
+
 :File:            ``addr_idx`` (rw)
 :Trace Registers: None.
 :Notes: