dt-bindings: Add doc for Pine64 Pinebook Pro

Add a compatible for Pine64 Pinebook Pro
Signed-off-by: Emmanuel Vadot <manu@freebsd.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200304213023.689983-2-t.schramm@manjaro.orgSigned-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -402,6 +402,11 @@ properties:
           - const: phytec,rk3288-phycore-som
           - const: rockchip,rk3288
 
+      - description: Pine64 PinebookPro
+        items:
+          - const: pine64,pinebook-pro
+          - const: rockchip,rk3399
+
       - description: Pine64 Rock64
         items:
           - const: pine64,rock64