x86: Use CONFIG_SERIAL_MULTI

Signed-off-by: Graeme Russ <graeme.russ@gmail.com>
diff --git a/include/serial.h b/include/serial.h
index 111edbe..6513d8e 100644
--- a/include/serial.h
+++ b/include/serial.h
@@ -26,7 +26,8 @@
 #if defined(CONFIG_405GP) || defined(CONFIG_405CR) || defined(CONFIG_440) || \
     defined(CONFIG_405EP) || defined(CONFIG_405EZ) || defined(CONFIG_405EX) || \
     defined(CONFIG_MPC5xxx) || defined(CONFIG_MPC83xx) || \
-    defined(CONFIG_MPC85xx) || defined(CONFIG_MPC86xx)
+    defined(CONFIG_MPC85xx) || defined(CONFIG_MPC86xx) || \
+    defined(CONFIG_SYS_SC520)
 extern struct serial_device serial0_device;
 extern struct serial_device serial1_device;
 #if defined(CONFIG_SYS_NS16550_SERIAL)