dt-bindings: display: rockchip-dsi: add optional #phy-cells property

The Rockchip DSI controller on some SoCs also controls a bidrectional
dphy, which would be connected to an Image Signal Processor as a phy
in the rx configuration.

So allow a #phy-cells property for the dsi controller.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Acked-by: Rob Herring <robh@kernel.org>
Tested-by: Sebastian Fricke <sebastian.fricke@posteo.net>
Acked-by: Helen Koike <helen.koike@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210210111020.2476369-3-heiko@sntech.de