net: axienet: implement NAPI and GRO receive

Implement NAPI and GRO receive. In addition to better performance, this
also avoids handling RX packets in hard IRQ context, which reduces the
IRQ latency impact to other devices.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Signed-off-by: David S. Miller <davem@davemloft.net>