net: lan966x: Add basic XDP support

Introduce basic XDP support to lan966x driver. Currently the driver
supports only the actions XDP_PASS, XDP_DROP and XDP_ABORTED.

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