[U-Boot] [U-Boot, v2, 3/3] rockchip: evb-rk3399: update document for board bring up

[Philipp Tomsich]

> 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 at rock-chips.com>
> Acked-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
> ---
> 
> Changes in v2:
> - typo fix, evb-firefly－>firefly-rk3399
> 
>  board/rockchip/evb_rk3399/README | 79 ++++++++++++++++++++++++++++++++--------
>  1 file changed, 63 insertions(+), 16 deletions(-)
> 

Reviewed-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>