coresight: Save active path for system tracers

coresight: Save active path for system tracers

This commit only set the path pointer for system tracers (e.g. STM) in
coresight_{enable|disable}_source().

Later changes will set the path pointer locally for per-CPU sources.
This is because the mode and path pointer must be set together, so that
they are observed atomically by the CPU PM notifier.

Reviewed-by: Yeoreum Yun <yeoreum.yun@arm.com>
Reviewed-by: James Clark <james.clark@linaro.org>
Tested-by: James Clark <james.clark@linaro.org>
Tested-by: Jie Gan <jie.gan@oss.qualcomm.com>
Signed-off-by: Leo Yan <leo.yan@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20260515-arm_coresight_path_power_management_improvement-v14-19-f88c4a3ecfe9@arm.com