Changed PPC4xx EMAC driver to require CONFIG_PPC4xx_EMAC

All in-tree IBM/AMCC PPC4xx boards using the EMAC get this new CONFIG

Signed-off-by: Ben Warren <biggerbadderben@gmail.com>
Acked-by: Stefan Roese <sr@denx.de>
diff --git a/include/configs/DU440.h b/include/configs/DU440.h
index 508a0ca..729153c 100644
--- a/include/configs/DU440.h
+++ b/include/configs/DU440.h
@@ -253,6 +253,7 @@
 int du440_phy_addr(int devnum);
 #endif
 
+#define CONFIG_PPC4xx_EMAC
 #define	CONFIG_IBM_EMAC4_V4	1
 #define CONFIG_MII		1	/* MII PHY management		*/
 #define CONFIG_PHY_ADDR		du440_phy_addr(0) /* PHY address	*/