.editorconfig: update the automatic mode setting for Emacs

It seems the editor specific keywords have been deprecated in the main
editorconfig plugin:

  https://github.com/editorconfig/editorconfig-emacs#file-type-file_type_ext-file_type_emacs



Update the keywords to the suggested one and point users at the
extension.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210305144839.6558-1-alex.bennee@linaro.org>