docs: dt: convert ABI.txt to ReST format

This file only requires a properly-formatted title to be
recognized as a ReST file.

As there will be more files under bindings/ that will be
included at the documentation body, add a new index.rst
file there.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>