arm64: dts: rockchip: Add missing secondary compatible for PX30 DSI

Add second DSI compatible to comply with DT schema validation
comming in the second patch.

Signed-off-by: David Heidelberg <david@ixit.cz>
Link: https://lore.kernel.org/r/20211211233818.88482-1-david@ixit.cz


Signed-off-by: Heiko Stuebner <heiko@sntech.de>