serial: uartlite: Support for SERIAL_MULTI

Add support for SERIAL MULTI for uartlite.

Signed-off-by: Michal Simek <monstr@monstr.eu>
diff --git a/include/serial.h b/include/serial.h
index ff1ce99..5926244 100644
--- a/include/serial.h
+++ b/include/serial.h
@@ -50,6 +50,13 @@
 extern struct serial_device serial6_device;
 #endif
 
+#if defined(CONFIG_XILINX_UARTLITE)
+extern struct serial_device uartlite_serial0_device;
+extern struct serial_device uartlite_serial1_device;
+extern struct serial_device uartlite_serial2_device;
+extern struct serial_device uartlite_serial3_device;
+#endif
+
 #if defined(CONFIG_S3C2410)
 extern struct serial_device s3c24xx_serial0_device;
 extern struct serial_device s3c24xx_serial1_device;