dt-bindings: phy: Fix the PX30 DSI PHY compatible in the example

Use the upstream compatible instead of the BSP one in the example
section of the DT bindings for this IP.

Fixes: 3817c796

 ("dt-bindings: phy: add yaml binding for rockchip,px30-dsi-dphy")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Rob Herring <robh@kernel.org>