perf tools: Rename group to topdown

perf tools: Rename group to topdown

The group.h/c only include TopDown group related functions. The name
"group" is too generic and inaccurate. Use the name "topdown" to replace
it.

Move topdown related functions to a dedicated file, topdown.c.

Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20200911144808.27603-2-kan.liang@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>