dt-bindings: add doc for Firefly ROC-RK3399-PC-PLUS

Add devicetree binding documentation for the Firefly ROC-RK3399-PC-PLUS.
Signed-off-by: Levin Du <djw@t-chip.com.cn>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210628035402.16812-2-djw@t-chip.com.cnSigned-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -127,6 +127,12 @@ properties:
               - firefly,roc-rk3399-pc-mezzanine
           - const: rockchip,rk3399
 
+      - description: Firefly ROC-RK3399-PC-PLUS
+        items:
+          - enum:
+              - firefly,roc-rk3399-pc-plus
+          - const: rockchip,rk3399
+
       - description: FriendlyElec NanoPi R2S
         items:
           - const: friendlyarm,nanopi-r2s