freq.py: calculate column widths dynamically

Some tracepoints have names longer than the fixed 15 characters which
results in a hard to read table. Let's adjust column width to the
tracepoint name length, with a minimum width of 15, as before.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>