dt-bindings: add rk3288-based Asus Tinker board

Tinker board is a credit card size develop board designed by Asus.
Powered by RK3288.
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NEddie Cai <eddie.cai.linux@gmail.com>
Signed-off-by: NHeiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.txt

 Rockchip platforms device tree bindings
 ---------------------------------------
+- Asus Tinker board
+    Required root node properties:
+      - compatible = "asus,rk3288-tinker", "rockchip,rk3288";
 
 - Kylin RK3036 board:
     Required root node properties: