dt-bindings: display: rockchip-dsi: add px30 compatible

The px30 SoC also uses a dw-mipi-dsi controller, so add the
compatible value for it.

changes in v5:
- rebased on top of 5.5-rc1
- merged with dsi timing change to prevent ordering conflicts

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191209143130.4553-6-heiko@sntech.de