rockchip: rv1108: Convert to OF_UPSTREAM

Instead of using the local rv1108 devicetree copies from U-Boot,
let's convert the rv1108 boards to OF_UPSTREAM so that the upstream kernel
devicetrees can be used instead.

Tested on a rv1108-elgin-r1 board.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Otavio Salvador <otavio@ossystems.com.br>
Tested-by: Otavio Salvador <otavio@ossystems.com.br>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>