[U-Boot] rockchip: doc: update U-Boot location info

[Philipp Tomsich]

> The U-Boot location has been moved to block 16384.
> This is 8MB, not 4MB.
> 
> Signed-off-by: Simon Goldschmidt <sgoldschmidt at de.pepperl-fuchs.com>
> Reviewed-by: Jagan Teki <jagan at openedev.com>
> Acked-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
> Reviewed-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
> ---
>  doc/README.rockchip | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 

Applied to u-boot-rockchip, thanks!