ixp: move serial to drivers/serial

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/include/configs/ixdpg425.h b/include/configs/ixdpg425.h
index 528bccd..193008e 100644
--- a/include/configs/ixdpg425.h
+++ b/include/configs/ixdpg425.h
@@ -72,6 +72,7 @@
 /* allow to overwrite serial and ethaddr */
 #define CONFIG_ENV_OVERWRITE
 
+#define CONFIG_IXP_SERIAL
 #define CONFIG_BAUDRATE         115200
 #define CONFIG_SYS_IXP425_CONSOLE	IXP425_UART1   /* we use UART1 for console */