Since we support ATF in SPL and add script for it, let's make the document up to date. Signed-off-by: Kever Yang <kever.yang@rock-chips.com> Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com> Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>