net: lan966x: add ethtool configuration and statistics

This patch adds support for statistics counters for the network
interfaces. Also adds support for configuring the network interface via
ethtool like: speed, duplex etc.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>