Add libps2 to the input section of driver-api

libps2 has been using kerneldoc to document its methods, but was not
actually plugged into driver-api.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/ZoMQhkyUQYi1Bx4t@google.com