perf top: update to use the new freq interface

Provide perf top -F as alternative to -c.

[ Impact: new 'perf top' feature ]

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <20090515132018.707922166@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>