Add fw_cfg DMA interface specification in the documentation.

Based on Gerd Hoffman's initial implementation.
Signed-off-by: NMarc Marí <markmb@redhat.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>