serial: Remove CONFIG_SERIAL_MULTI from remaining sources

Remove the parts depending either on disabled CONFIG_SERIAL_MULTI
or ifdefs around CONFIG_SERIAL_MULTI parts since CONFIG_SERIAL_MULTI
is now enabled by default.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Stefan Roese <sr@denx.de>
diff --git a/include/serial.h b/include/serial.h
index 01a86b4..a8d23f5 100644
--- a/include/serial.h
+++ b/include/serial.h
@@ -63,7 +63,7 @@
 
 #endif /* CONFIG_USB_TTY */
 
-#if defined(CONFIG_MPC512X) &&  defined(CONFIG_SERIAL_MULTI)
+#if defined(CONFIG_MPC512X)
 extern struct stdio_dev *open_port(int num, int baudrate);
 extern int close_port(int num);
 extern int write_port(struct stdio_dev *port, char *buf);