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/korat.h b/include/configs/korat.h
index ca3e8a9..d56da14 100644
--- a/include/configs/korat.h
+++ b/include/configs/korat.h
@@ -215,6 +215,7 @@
 #define CONFIG_LOADS_ECHO	1	/* echo on for serial download	*/
 #define CONFIG_SYS_LOADS_BAUD_CHANGE	1	/* allow baudrate change	*/
 
+#define CONFIG_PPC4xx_EMAC
 #define CONFIG_IBM_EMAC4_V4	1
 #define CONFIG_MII		1	/* MII PHY management		*/
 #define CONFIG_PHY_ADDR		2	/* PHY address, See schematics	*/