dt-bindings: add root compatible property for PopMetal board

PopMetal board is a rk3288 based board made by ChipSpark, this
add root compatible property for it
Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.txt

@@ -22,3 +22,7 @@ Rockchip platforms device tree bindings
       - compatible = "firefly,firefly-rk3288", "rockchip,rk3288";
     or
       - compatible = "firefly,firefly-rk3288-beta", "rockchip,rk3288";
+
+- ChipSPARK PopMetal-RK3288 board:
+    Required root node properties:
+      - compatible = "chipspark,popmetal-rk3288", "rockchip,rk3288";