net: ethernet: am65-cpsw: Add standard Ethernet MAC stats to ethtool

Gets 'ethtool -S eth0 --groups eth-mac' command to work.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>