coresight: Move CPU hotplug callbacks to core layer

This commit moves CPU hotplug callbacks from ETMv4 driver to core layer.
The motivation is the core layer can control all components on an
activated path rather but not only managing tracer in ETMv4 driver.

The perf event layer will disable CoreSight PMU event 'cs_etm' when
hotplug off a CPU.  That means a perf mode will be always converted to
disabled mode in CPU hotplug.  Arm CoreSight CPU hotplug callbacks only
need to handle the Sysfs mode and ignore the perf mode.

Add a 'mode' argument to coresight_pm_get_active_path() so it only
returns active paths for the relevant mode. Define the enum with bit
flags so it is safe for bitwise operations.

Change CPUHP_AP_ARM_CORESIGHT_STARTING to CPUHP_AP_ARM_CORESIGHT_ONLINE
so that the CPU hotplug callback runs in the online state and thread
context, allowing coresight_disable_sysfs() to be called directly to
disable the path.

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-v14-27-f88c4a3ecfe9@arm.com

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