TTY: nfcon, add tty_port and link it

Every tty driver needs tty_port for each line. So let us add one to
nfcon too. And link it so that the tty layer knows about it.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-m68k@lists.linux-m68k.org
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>