dt-bindings: display: rockchip: add document for px30 vop

Add the compatible values for the px30 display controllers.

Signed-off-by: Sandy Huang <hjc@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
[added commit message]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1530003215-46593-2-git-send-email-hjc@rock-chips.com