net: macb: Add SG support for Zynq SOC family

Enable SG support for Zynq SOC family devices.

Signed-off-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>