net: dsa: mv88e6xxx: Add "rmon" counter group support

Report the applicable subset of an mv88e6xxx port's counters using
ethtool's standardized "rmon" counter group.

Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: David S. Miller <davem@davemloft.net>