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

Documentation/devicetree/bindings/display/rockchip/rockchip-vop.txt

@@ -8,6 +8,8 @@ Required properties:
 - compatible: value should be one of the following
 		"rockchip,rk3036-vop";
 		"rockchip,rk3126-vop";
+		"rockchip,px30-vop-lit";
+		"rockchip,px30-vop-big";
 		"rockchip,rk3288-vop";
 		"rockchip,rk3368-vop";
 		"rockchip,rk3366-vop";