rockchip: evb-rk3399: update document for board bring up

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>