net: mvpp2: add ethtool GOP statistics

Add ethtool statistics support by reading the GOP statistics from the
hardware counters. Also implement a workqueue to gather the statistics
every second or some 32-bit counters could overflow.

Suggested-by: Stefan Chulski <stefanc@marvell.com>
Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>