rockchip: ringneck-px30: enable DT overlay support

rockchip: ringneck-px30: enable DT overlay support

Haikou carrierboard allows multiple adapter boards to be connected, for
now there exists the following adapter boards compatible with PX30
Ringneck:
 - Haikou Video Demo on the Video Connector,
 - Haikou LVDS 9904379 on the Video Connector,

So support DT overlays so we can use this mechanism instead of full DTB
containing both the carrierboard and the adapter.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>