ixp: move serial to drivers/serial

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/include/configs/pdnb3.h b/include/configs/pdnb3.h
index f8aac1a..4da401f 100644
--- a/include/configs/pdnb3.h
+++ b/include/configs/pdnb3.h
@@ -68,6 +68,7 @@
 /* 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 */