Documentation: rapidio: move sysfs interface to ABI

Right now, the description of the rapidio sysfs interfaces is in
Documentation/rapidio/sysfs.txt. Since these are a part of the ABI, they
should be in Documentation/ABI along with the rest.

Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Acked-by: Alexandre Bounine <alexandre.bounine@idt.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>