[U-Boot] [U-Boot, 4/4] doc: rockchip: Add global doc for rk3399 build/run

[Philipp Tomsich]

> Since rockchip have an individual doc/README.rockchip, it would
> be better to update the same instead of maintaining it separately
> in board files.
> 
> So, add the documentation for rk3399
> - procedure to build
> - procedure to boot from SD
> 
> Signed-off-by: Jagan Teki <jagan at amarulasolutions.com>
> ---
>  doc/README.rockchip | 69 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 69 insertions(+)
> 

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