Add fw_cfg DMA interface specification in the documentation.
Based on Gerd Hoffman's initial implementation.
Signed-off-by: Marc Marí <markmb@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>