[U-Boot] rockchip: dts: rk3399-firely: add 'same-as-spl'

[Philipp Tomsich]

> Like on rk3399-puma we want to continue booting the fill U-Boot from
> the same device as the SPL stage.
> 
> Signed-off-by: Mark Kettenis <kettenis at openbsd.org>
> ---
>  arch/arm/dts/rk3399-firefly.dts | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 

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