perf report: Parse time quantum

Many workloads change over time. 'perf report' currently aggregates the
whole time range reported in perf.data.

This patch adds an option for a time quantum to quantisize the perf.data
over time.

This just adds the option, will be used in follow on patches for a time
sort key.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20190305144758.12397-6-andi@firstfloor.org


[ Use NSEC_PER_[MU]SEC ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>