Add a placeholder doc for the exynos4 boards nuri and smdkc210.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Message-id: 20241018141332.942844-5-peter.maydell@linaro.org