coresight: Take hotplug lock in enable_source_store() for Sysfs mode

The hotplug lock is acquired and released in etm{3|4}_disable_sysfs(),
which are low-level functions.  This prevents us from a new solution for
hotplug.

Firstly, hotplug callbacks cannot invoke etm{3|4}_disable_sysfs() to
disable the source; otherwise, a deadlock issue occurs.  The reason is
that, in the hotplug flow, the kernel acquires the hotplug lock before
calling callbacks.  Subsequently, if coresight_disable_source() is
invoked and it calls etm{3|4}_disable_sysfs(), the hotplug lock will be
acquired twice, leading to a double lock issue.

Secondly, when hotplugging a CPU on or off, if we want to manipulate all
components on a path attached to the CPU, we need to maintain atomicity
for the entire path.  Otherwise, a race condition may occur with users
setting the same path via the Sysfs knobs, ultimately causing mess
states in CoreSight components.

This patch moves the hotplug locking from etm{3|4}_disable_sysfs() into
enable_source_store().  As a result, when users control the Sysfs knobs,
the whole flow is protected by hotplug locking, ensuring it is mutual
exclusive with hotplug callbacks.

Note, the paired function etm{3|4}_enable_sysfs() does not use hotplug
locking, which is why this patch does not modify it.

Reviewed-by: Mike Leach <mike.leach@linaro.org>
Tested-by: James Clark <james.clark@linaro.org>
Reviewed-by: Yeoreum Yun <yeoreum.yun@arm.com>
Reviewed-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-v14-6-f88c4a3ecfe9@arm.com

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