[U-Boot] [U-Boot, v2] rockchip: doc: update latest info to document

[Philipp Tomsich]

> - Add some rk3399 and rk3328 boards;
> - use rkdeveloptool;
> - add link to opensource.rock-chips.com instead of wikidot;
> - other update.
> 
> Signed-off-by: Kever Yang <kever.yang at rock-chips.com>
> ---
> 
> Changes in v2:
> - add rkflashtool info back
> 
>  doc/README.rockchip | 48 ++++++++++++++++++++++++++++--------------------
>  1 file changed, 28 insertions(+), 20 deletions(-)
> 

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