dt-bindings: document rockchip rk3399-evb board

Use "rockchip,rk3399-evb" compatible string for Rockchip RK3399
evaluation board.
Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.txt

@@ -101,4 +101,8 @@ Rockchip platforms device tree bindings
 
 - Rockchip RK3228 Evaluation board:
     Required root node properties:
-      - compatible = "rockchip,rk3228-evb", "rockchip,rk3228";
+     - compatible = "rockchip,rk3228-evb", "rockchip,rk3228";
+
+- Rockchip RK3399 evb:
+    Required root node properties:
+      - compatible = "rockchip,rk3399-evb", "rockchip,rk3399";