[U-Boot] rockchp: dts: rk3399-evb: support boot from sd-card

[Philipp Tomsich]

> Enable sdmmc node in SPL and add it to boot order.
> 
> Signed-off-by: Kever Yang <kever.yang at rock-chips.com>
> Acked-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
> ---
> 
>  arch/arm/dts/rk3399-evb.dts | 3 +++
>  1 file changed, 3 insertions(+)
> 

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