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

Documentation/devicetree/bindings/display/rockchip/dw_mipi_dsi_rockchip.txt

@@ -23,6 +23,7 @@ Required properties:
 Optional properties:
 - phys: from general PHY binding: the phandle for the PHY device.
 - phy-names: Should be "dphy" if phys references an external phy.
+- #phy-cells: Defined when used as ISP phy, should be 0.
 - power-domains: a phandle to mipi dsi power domain node.
 - resets: list of phandle + reset specifier pairs, as described in [3].
 - reset-names: string reset name, must be "apb".