net: atlantic: implement UDP GSO offload

atlantic hardware does support UDP hardware segmentation offload.
This allows user to specify one large contiguous buffer with data
which then will be split automagically into multiple UDP packets
of specified size.

Bulk sending of large UDP streams lowers CPU usage and increases
bandwidth.

We did estimations both with udpgso_bench_tx test tool and with modified
iperf3 measurement tool (4 streams, multithread, 200b packet size)
over AQC<->AQC 10G link. Flow control is disabled to prevent RX side
impact on measurements.

No UDP GSO:
	iperf3 -c 10.0.1.2 -u -b0 -l 200 -P4 --multithread
UDP GSO:
	iperf3 -c 10.0.1.2 -u -b0 -l 12600 --udp-lso 200 -P4 --multithread

Mode          CPU   iperf speed    Line speed   Packets per second
-------------------------------------------------------------
NO UDP GSO    350%   3.07 Gbps      3.8 Gbps     1,919,419
SW UDP GSO    200%   5.55 Gbps      6.4 Gbps     3,286,144
HW UDP GSO    90%    6.80 Gbps      8.4 Gbps     4,273,117

Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>