net: mvneta: add support for page_pool_get_stats

Introduce support for the page_pool stats API into mvneta driver.
Report page_pool stats through ethtool.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>