coresight: tmc: Add missing doc including reading and etr_mode of struct tmc_drvdata

tmc_drvdata::reading is used to indicate whether a reading process
is performed through /dev/xyz.tmc.
tmc_drvdata::etr_mode is used to store the Coresight TMC-ETR buffer
mode selected by the user.
Document them.

Reviewed-by: James Clark <james.clark@linaro.org>
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
Reviewed-by: Leo Yan <leo.yan@arm.com>
Signed-off-by: Junhao He <hejunhao3@h-partners.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20260121101543.2017014-2-wangyushan12@huawei.com

drivers/hwtracing/coresight/coresight-tmc.h

@@ -221,6 +221,7 @@ struct tmc_resrv_buf {
  * @pid:	Process ID of the process that owns the session that is using
  *		this component. For example this would be the pid of the Perf
  *		process.
+ * @reading:	buffer's in the reading through "/dev/xyz.tmc" entry
  * @stop_on_flush: Stop on flush trigger user configuration.
  * @buf:	Snapshot of the trace data for ETF/ETB.
  * @etr_buf:	details of buffer used in TMC-ETR
@@ -233,6 +234,7 @@ struct tmc_resrv_buf {
  * @trigger_cntr: amount of words to store after a trigger.
  * @etr_caps:	Bitmask of capabilities of the TMC ETR, inferred from the
  *		device configuration register (DEVID)
+ * @etr_mode:	User preferred mode of the ETR device, default auto mode.
  * @idr:	Holds etr_bufs allocated for this ETR.
  * @idr_mutex:	Access serialisation for idr.
  * @sysfs_buf:	SYSFS buffer for ETR.