reproduce:
        git remote add stefandoesinger-zx297520 https://gitlab.com/stefandoesinger/zx297520-kernel
        git fetch --no-tags stefandoesinger-zx297520 mfd
        git checkout 1e6a3951315be7aa7acb8380b0bb39dde1507e4b
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # pip install -r Documentation/sphinx/requirements.txt
        make htmldocs