net: mvneta: add ethtool statistics

Add support for the ethtool statistic interface, returning the full set
of statistics which both Armada 370, 38x and Armada XP can support.

Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>