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

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