net/mlx5e: Add per priority group to PPort counters

Expose counters providing information for each priority level (PCP) through
ethtool -S option and DCBNL.
This includes rx/tx bytes, frames, and pause counters.

Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>