net/macb: add config for Atmel sama5d2 SoCs

Add the compatible string for Atmel sama5d2 SoC family as the configuration
options differ from other instances of the GEM.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>