drm/rockchip: dsi: add px30 support

Add the compatible and GRF definitions for the PX30 soc.

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>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191209143130.4553-7-heiko@sntech.de