Documentation: tty: move n_gsm to tty

We have n_tty documented in Documentation/driver-api/tty/. n_gsm belongs
there too, so move from serial/ to tty/ too.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220411110143.10019-4-jslaby@suse.cz


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