Add line item reference to quanta-gbs-bmc machine.

Signed-off-by: Patrick Venture <venture@google.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20210615192848.1065297-3-venture@google.com
[PMM: fixed underline Sphinx warning]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>