[U-Boot] rockchip: configs: use rockchip-common.h for rk3368

[Philipp Tomsich]

> rockchip-common.h already defines values that are missing from
> rk3368_common.h
> 
> For example BOOT_TARGET_DEVICES was defined empty and therefore
> distroboot had no boot targets.
> 
> Signed-off-by: Klaus Goger <klaus.goger at theobroma-systems.com>
> ---
> 
>  include/configs/rk3368_common.h | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 

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