Documentation: move tty to driver-api

Based on discussion starting as 87mthw2o93.fsf@meer.lwn.net, let's move
the tty documentation to driver-api. It's more appropriate there.

Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220411110143.10019-2-jslaby@suse.cz


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