docs: conf.py: fix c:function support with Sphinx 3.x

Sphinx C parser for the C domain is now more pedantic when
trying to identify the function types. That prevents scope
macros to be used as type defines.

Yet, since 3.0.2, it is possible to provide it a list of
such macros. Add them, in order to solve several Sphinx 3.x
warnings.

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