dt-bindings: Add doc for rk3308-evb

Add compatible for RK3308 Evaluation board
Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191021084642.28562-1-andy.yan@rock-chips.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -464,6 +464,11 @@ properties:
               - rockchip,rk3288-evb-rk808
           - const: rockchip,rk3288
 
+      - description: Rockchip RK3308 Evaluation board
+        items:
+          - const: rockchip,rk3308-evb
+          - const: rockchip,rk3308
+
       - description: Rockchip RK3328 Evaluation board
         items:
           - const: rockchip,rk3328-evb