perf_counter: create Documentation/perf_counter/ and move perfcounters.txt there

We'll have more files in that directory, prepare for that.
Acked-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>