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/PLU405.h b/include/configs/PLU405.h
index 675dbe6..11ce008 100644
--- a/include/configs/PLU405.h
+++ b/include/configs/PLU405.h
@@ -54,6 +54,7 @@
 #define CONFIG_NET_MULTI	1
 #undef  CONFIG_HAS_ETH1
 
+#define CONFIG_PPC4xx_EMAC
 #define CONFIG_MII		1	/* MII PHY management		*/
 #define CONFIG_PHY_ADDR		0	/* PHY address			*/
 #define CONFIG_LXT971_NO_SLEEP  1       /* disable sleep mode in LXT971 */