rtla/timerlat: Add a summary for top mode

While the per-cpu values are the results to take into consideration, the
overall system values are also useful.

Add a summary at the bottom of rtla timerlat top showing the overall
results. For instance:

                                       Timer Latency
    0 00:00:10   |          IRQ Timer Latency (us)        |         Thread Timer Latency (us)
  CPU COUNT      |      cur       min       avg       max |      cur       min       avg       max
    0 #10003     |      113        19       150       441 |      134        35       170       459
    1 #10003     |       63         8        99       462 |       84        15       119       481
    2 #10003     |        3         2        89       396 |       21         8       108       414
    3 #10002     |      206        11       210       394 |      223        21       228       415
  ---------------|----------------------------------------|---------------------------------------
  ALL #40011  e0 |                  2       137       462 |                  8       156       481

Link: https://lkml.kernel.org/r/5eb510d6faeb4ce745e09395196752df75a2dd1a.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>