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/csb272.h b/include/configs/csb272.h
index 393e992..874f2c0 100644
--- a/include/configs/csb272.h
+++ b/include/configs/csb272.h
@@ -180,6 +180,7 @@
  * MII PHY configuration
  *
  */
+#define CONFIG_PPC4xx_EMAC
 #define CONFIG_MII		1	/* MII PHY management		*/
 #define CONFIG_PHY_ADDR		0	/* PHY address			*/
 #define CONFIG_PHY_CMD_DELAY	40	/* PHY COMMAND delay		*/