arm/km: add piggy mac adress offset for mgcoge3un

On mgcoge3un the piggy mac adress is at offset 3.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Gerlando Falauto <gerlando.falauto@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>

include/configs/mgcoge3un.h

@@ -77,6 +77,7 @@
 	MVGBE_SET_MII_SPEED_TO_100)
 
 #define CONFIG_KM_BOARD_EXTRA_ENV	"waitforne=true\0"
+#define CONFIG_PIGGY_MAC_ADRESS_OFFSET  3
 
 /*
  * PCIe port not used on mgcoge3un