perf evlist: Be more specific on -F/--freq

Currently perf evlist -F shows the number as if it's always sampling
frequency.  But we now support per-event freq/period settings.  So it'd
better to show more detailed info whether it's freq or period.

  $ perf record -e 'cpu/config=1/,cpu/config=2,period=300000/' sleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.017 MB perf.data ]

  $ perf evlist -F
  cpu/config=1/: sample_freq=4000
  cpu/config=2,period=300000/: sample_period=300000

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1439102724-14079-2-git-send-email-namhyung@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>