rockchip: add px30-evb board

The px30 evb is an evaluation board for the px30 together with a dsi-
connected display. This adds board and config files for it.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>