net: macb: Disable half duplex gigabit on Zynq

According to the Zynq TRM, gigabit half duplex is not supported.  Add a
new cap and compatible string so Zynq can avoid advertising that mode.

Signed-off-by: Nathan Sullivan <nathan.sullivan@ni.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>