net: enetc: expose some standardized ethtool counters

Structure the code in such a way that it can be reused later for the
pMAC statistics, by just changing the "mac" argument to 1.

Usage:
ethtool --include-statistics --show-pause eno2
ethtool -S eno0 --groups eth-mac
ethtool -S eno0 --groups eth-ctrl
ethtool -S eno0 --groups rmon

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>