perf bench: List output formatting options on 'perf bench -h'

So 'perf bench -h' is not very helpful when printing the help line
about the output formatting options:

    -f, --format <default>
                              Specify format style

There are two output format styles, 'default' and 'simple', so improve
the help text to:

    -f, --format <default|simple>
                              Specify the output formatting style

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1445241870-24854-7-git-send-email-mingo@kernel.org


[ Removed leftovers from the mem-functions.c rename ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>