perf Documentation: Describe the PMU naming convention

perf Documentation: Describe the PMU naming convention

It is an existing convention to use suffixes with PMU names. Try to
capture that convention so that future PMU devices may adhere to it.

The name of the file and date within the file try to follow existing
conventions, particularly sysfs-bus-event_source-devices-events.

Reviewed-by: James Clark <james.clark@arm.com>
Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Reviewed-by: Leo Yan <leo.yan@arm.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Bharat Bhushan <bbhushan2@marvell.com>
Cc: Bhaskara Budiredla <bbudiredla@marvell.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Tuan Phan <tuanphan@os.amperecomputing.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20240606044959.335715-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>