coresight: docs: Document etm4x timestamp interval option

Document how the new field is used, maximum value and the interaction
with SYNC timestamps.

Tested-by: Leo Yan <leo.yan@arm.com>
Signed-off-by: James Clark <james.clark@linaro.org>
Reviewed-by: Leo Yan <leo.yan@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20251128-james-cs-syncfreq-v8-13-4d319764cc58@linaro.org

Documentation/trace/coresight/coresight.rst

@@ -613,8 +613,20 @@ They are also listed in the folder /sys/bus/event_source/devices/cs_etm/format/
      - Session local version of the system wide setting: :ref:`ETM_MODE_RETURNSTACK
        <coresight-return-stack>`
    * - timestamp
-     - Session local version of the system wide setting: :ref:`ETMv4_MODE_TIMESTAMP
-       <coresight-timestamp>`
+     - Controls generation and interval of timestamps.
+
+       0 = off, 1 = minimum interval .. 15 = maximum interval.
+
+       Values 1 - 14 use a counter that decrements every cycle to generate a
+       timestamp on underflow. The reload value for the counter is 2 ^ (interval
+       - 1). If the value is 1 then the reload value is 1, if the value is 11
+       then the reload value is 1024 etc.
+
+       Setting the maximum interval (15) will disable the counter generated
+       timestamps, freeing the counter resource, leaving only ones emitted when
+       a SYNC packet is generated. The sync interval is controlled with
+       TRCSYNCPR.PERIOD which is every 4096 bytes of trace by default.
+
    * - cc_threshold
      - Cycle count threshold value. If nothing is provided here or the provided value is 0, then the
        default value i.e 0x100 will be used. If provided value is less than minimum cycles threshold