simpleperf: add rate comment for new raw events.

After the patch, simpleperf shows rate comments for new raw events:

$ simpleperf stat --group raw-l1d-cache-refill-rd,raw-l1d-cache-rd sleep 1
Performance counter statistics:

     98,519  raw-l1d-cache-refill-rd   # 2.226422% level 1 cache refill rate, read  (100%)
  4,424,993  raw-l1d-cache-rd          #                                            (100%

Bug: 137044139
Test: run simpleperf_unit_test.
Test: run simpleperf manually.
Change-Id: Ic71ee7326451e62feefb26feb0471fa3a72a173f
2 files changed