docs: automarkup.py: Use new C roles in Sphinx 3

While Sphinx 2 used a single c:type role for struct, union, enum and
typedef, Sphinx 3 uses a specific role for each one.
To keep backward compatibility, detect the Sphinx version and use the
correct roles for that version.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@protonmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>