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/EXBITGEN.h b/include/configs/EXBITGEN.h
index 9f5d3ca..1dd6e57 100644
--- a/include/configs/EXBITGEN.h
+++ b/include/configs/EXBITGEN.h
@@ -88,6 +88,7 @@
  */
 #define CONFIG_BOOTP_BOOTFILESIZE
 #define CONFIG_BOOTP_BOOTPATH
+#define CONFIG_PPC4xx_EMAC
 #define CONFIG_BOOTP_GATEWAY
 #define CONFIG_BOOTP_HOSTNAME