perf tests: test_arm_coresight: Simplify source iteration

There are two reasons to do this, firstly there is a shellcheck warning
in cs_etm_dev_name(), which can be completely deleted. And secondly the
current iteration method doesn't support systems with both ETE and ETM
because it picks one or the other. There isn't a known system with this
configuration, but it could happen in the future.

Iterating over all the sources for each CPU can be done by going through
/sys/bus/event_source/devices/cs_etm/cpu* and following the symlink back
to the Coresight device in /sys/bus/coresight/devices. This will work
whether the device is ETE, ETM or any future name, and is much simpler
and doesn't require any hard coded version numbers

Suggested-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: James Clark <james.clark@arm.com>
Acked-by: Ian Rogers <irogers@google.com>
Tested-by: Leo Yan <leo.yan@linaro.org>
Cc: tianruidong@linux.alibaba.com
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Anushree Mathur <anushree.mathur@linux.vnet.ibm.com>
Cc: Tiezhu Yang <yangtiezhu@loongson.cn>
Cc: atrajeev@linux.vnet.ibm.com
Cc: coresight@lists.linaro.org
Link: https://lore.kernel.org/r/20231023131550.487760-1-james.clark@arm.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

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