selftests: txtimestamp: print statistics for timestamp events.

Statistics on timestamps is useful to quantify average and tail latency.

Print timestamp statistics in count/avg/min/max format.

Signed-off-by: Jian Yang <jianyang@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>