arm64: dts: rockchip: add cdn-dp node for rk3399.

Add a node for the cdn DP controller which is embedded in the rk3399
SoC.

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
[fixed whitespaces instead of tabs, dropped unnecessary address+size-cells
 and fixed the number of interrupt cells]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>