dt-bindings: display: rockchip-lvds: Document PX30 PHY

PX30 SoCs use a single PHY shared by two display pipelines: MIPI DSI
and LVDS. In the case of the LVDS IP, document the possibility to fill
a PHY handle.

Signed-off-by: Miquel Raynal <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Heiko Stuebner <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/devicetree/bindings/display/rockchip/rockchip-lvds.txt

···
 - rockchip,grf: phandle to the general register files syscon
 - rockchip,output: "rgb", "lvds" or "duallvds", This describes the output interface
 
+- phys: LVDS/DSI DPHY (px30 only)
+- phy-names: name of the PHY, must be "dphy" (px30 only)
+
 Optional properties:
 - pinctrl-names: must contain a "lcdc" entry.
 - pinctrl-0: pin control group to be used for this controller.