serial: bfin: Remove the bfin_serialN_device exports from serial.h

Remove the exports from serial.h as they are only used in the blackfin
serial driver. Furthermore, they are only used for registration, which
is handled already inside that driver and default_serial_port() call,
which is also handled in that driver.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>
Cc: Mike Frysinger <vapier@gentoo.org>

include/serial.h

@@ -48,10 +48,6 @@ extern struct serial_device zoom2_serial_device3;
 
 #if defined(CONFIG_SYS_BFIN_UART)
 extern void serial_register_bfin_uart(void);
-extern struct serial_device bfin_serial0_device;
-extern struct serial_device bfin_serial1_device;
-extern struct serial_device bfin_serial2_device;
-extern struct serial_device bfin_serial3_device;
 #endif
 
 extern void serial_register(struct serial_device *);