dt-bindings: display: rockchip-lvds: Declare PX30 compatible

Document the PX30 LVDS compatible.

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/20191224143900.23567-2-miquel.raynal@bootlin.com