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 <miquel.raynal@bootlin.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20191224143900.23567-3-miquel.raynal@bootlin.com