dt-bindings: add document for rockchip dp phy

Add dt binding documentation for rockchip display port PHY.

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