arm64: dts: rockchip: add dsi controller for px30

This adds the dw-mipi-dsi controller and hooks it into the
display-subsystem on px30.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Link: https://lore.kernel.org/r/20200106112005.795834-1-heiko@sntech.de