simpleperf: Suport "pmu" type events in list command

Support low-level PMU counters to be listed using "pmu" argument.

Test: run simpleperf_unit_test
Change-Id: I7bc8251a4fa12bae82142b71afa70c359ad24d40
Signed-off-by: Namhyung Kim <namhyung@google.com>
3 files changed