tty/serial: fix apbuart build

Fix build errors by selecting SERIAL_CORE:

ERROR: "uart_register_driver" [drivers/tty/serial/apbuart.ko] undefined!
ERROR: "uart_write_wakeup" [drivers/tty/serial/apbuart.ko] undefined!
ERROR: "uart_update_timeout" [drivers/tty/serial/apbuart.ko] undefined!
ERROR: "uart_get_divisor" [drivers/tty/serial/apbuart.ko] undefined!
ERROR: "uart_get_baud_rate" [drivers/tty/serial/apbuart.ko] undefined!
ERROR: "uart_add_one_port" [drivers/tty/serial/apbuart.ko] undefined!
ERROR: "uart_unregister_driver" [drivers/tty/serial/apbuart.ko] undefined!
ERROR: "uart_remove_one_port" [drivers/tty/serial/apbuart.ko] undefined!

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Daniel Hellstrom <daniel@gaisler.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>