Documentation: tty: move moxa-smartio.rst to tty

It is a tty_driver, not serial -- uart_driver. So move it to the tty
docs dir too. (The same as the driver itself.)

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220411110143.10019-5-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>