dt-bindings: add document for rk3126-vop

Signed-off-by: Sandy Huang <hjc@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1510658855-90223-1-git-send-email-hjc@rock-chips.com

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

@@ -7,6 +7,7 @@ buffer to an external LCD interface.
 Required properties:
 - compatible: value should be one of the following
 		"rockchip,rk3036-vop";
+		"rockchip,rk3126-vop";
 		"rockchip,rk3288-vop";
 		"rockchip,rk3368-vop";
 		"rockchip,rk3366-vop";