mlxsw: reg: Add ecn_marked_tc to Per-TC Congestion Counters

The PPCNT register retrieves per port performance counters. The
ecn_marked_tc field in per-TC Congestion counter group contains a count of
packets marked as ECN or potentially marked as ECN.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>