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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/driver-api/serial/index.rst

···
 .. toctree::
     :maxdepth: 1
 
-    moxa-smartio
     serial-iso7816
     serial-rs485
 

Documentation/driver-api/tty/index.rst

···
 .. toctree::
    :maxdepth: 2
 
+   moxa-smartio
    n_gsm
    n_tty