perf tools: Add overhead/overhead_children keys defaults via string

We currently set 'overhead' and 'overhead_children' as default sort keys
within perf_hpp__init function by directly adding into the sort list.

This patch adds 'overhead' and 'overhead_children' in text form into
sort_keys and let them be added by standard sort dimension interface.

We need to eliminate dirrect sort_list additions to be able to add
support for hists specific sort keys.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Noel Grandin <noelgrandin@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1452158050-28061-12-git-send-email-jolsa@kernel.org


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