ARM: dts: at91: add new compatibility string for macb on sama5d3

We need this new compatibility string as we experienced different behavior
for this 10/100Mbits/s macb interface on this particular SoC.
Backward compatibility is preserved as we keep the alternative strings.

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