rtla/timerlat: Add a summary for hist mode

Like on rtla timerlat top, add an overall summary at the bottom
of timerlat hist. For instance:

  # timerlat hist -c 0-1 -d 10s -E 20
  # RTLA timerlat histogram
  # Time unit is microseconds (us)
  # Duration:   0 00:00:10
  Index   IRQ-000   Thr-000   IRQ-001   Thr-001
  6             1         0         0         0
  7             1         0         0         0
  8             1         0         1         0
  9             7         0         0         0
  10           16         0         0         0
  11            1         0         3         0
  15            0         0         3         0
  16            0         0        12         0
  17            0         0        28         0
  18            0         2        26         0
  19            1         1        80         1
  over:      9973      9998      9848     10000
  count:    10001     10001     10001     10001
  min:          6        18         8        19
  avg:        185       204        95       113
  max:        428       450       341       371
  ALL:        IRQ       Thr
  count:    20002     20002
  min:          6        18
  avg:        140       159
  max:        428       450

Link: https://lkml.kernel.org/r/a6bc06c798f72127edc57d1f99da8d57e1187cee.1713968967.git.bristot@kernel.org



Cc: Jonathan Corbet <corbet@lwn.net>
Suggested-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>