Documentation: bindings: add dt documentation for cdn DP controller

This patch adds a binding that describes the cdn DP controller for
rk3399.

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
[document the missing 3 reset-names]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20180213143549.15197-1-enric.balletbo@collabora.com