perf: Add timechart help text and add timechart to "perf help"

As suggested by Ingo, add a timechart man page help text, as well
as add it to the "perf help" overview.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Cc: fweisbec@gmail.com
Cc: peterz@infradead.org
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <20090919133604.3767fa35@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>