docs: gpio: convert docs to ReST and rename to *.rst

The API described at sysfs.txt is deprecated.

Still, as it is still part of the Kernel (and will likely be
there for some time, as we don't simply remove APIs). So,
it makes sense to keep it there.

The conversion of this file is trivial:
  - add blank lines and identation in order to identify paragraphs;
  - mark a literal block as such.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>