doc: rockchip: Add phyCORE-RK3288 RDK to board list

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Acked-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>