dt-bindings: Add doc for Firefly ROC-RK3308-CC board

Add compatible for Firefly ROC-RK3308-CC board.
Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Link: https://lore.kernel.org/r/20191030072648.29738-1-andy.yan@rock-chips.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -82,6 +82,11 @@ properties:
           - const: firefly,firefly-rk3399
           - const: rockchip,rk3399
 
+      - description: Firefly ROC-RK3308-CC
+        items:
+          - const: firefly,roc-rk3308-cc
+          - const: rockchip,rk3308
+
       - description: Firefly roc-rk3328-cc
         items:
           - const: firefly,roc-rk3328-cc