enetc: Add ethtool statistics

This adds most h/w statistics counters: non-privileged SI conters, as
well as privileged Port and MAC counters available only to the PF.
Per ring software stats are also included.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>