net: macb: disable scatter-gather for macb on sama5d3

Create a new configuration for the sama5d3-macb new compatibility string.
This configuration disables scatter-gather because we experienced lock down
of the macb interface of this particular SoC under very high load.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>