dt-bindings: add document for rockchip variant of analogix_dp

Rockchip DP driver is a helper driver of analogix_dp coder driver,
so most of the DT property should be descriped in analogix_dp document.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Yakir Yang <ykk@rock-chips.com>