dpaa2-eth: add ethtool MAC counters

When a DPNI is connected to a MAC, export its associated counters.
Ethtool related functions are added in dpaa2_mac for returning the
number of counters, their strings and also their values.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>