[U-Boot] [U-Boot, v1, 2/4] spl: document 'chosen/u-boot, spl-boot-device'

[Philipp Tomsich]

> To let the full U-Boot know where it was booted from (i.e. which of
> the entries in /chosen/u-boot,spl-boot-order' contained a valid
> image), we define (and document) /chosen/u-boot,spl-boot-device as the
> property that could/should automatically be injected by SPL.
> 
> This commit only contains a documentation change, which documents the
> new property and the intended usage.
> 
> Signed-off-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
> Tested-by: Klaus Goger <klaus.goger at theobroma-systems.com>
> ---
> 
>  doc/device-tree-bindings/chosen.txt | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 

Applied to u-boot-rockchip, thanks!