dt-bindings: Document rk3399 Gru/Kevin

Gru is a base dev board for a family of devices, including Kevin. Both
utilize Rockchip RK3399, and they share much of their design.
Signed-off-by: Brian Norris <briannorris@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
[added Samsung Chromebook Plus hint]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.txt

@@ -56,6 +56,17 @@ Rockchip platforms device tree bindings
       - compatible = "google,veyron-brain-rev0", "google,veyron-brain",
 		     "google,veyron", "rockchip,rk3288";
 
+- Google Gru (dev-board):
+    Required root node properties:
+      - compatible = "google,gru-rev15", "google,gru-rev14",
+		     "google,gru-rev13", "google,gru-rev12",
+		     "google,gru-rev11", "google,gru-rev10",
+		     "google,gru-rev9", "google,gru-rev8",
+		     "google,gru-rev7", "google,gru-rev6",
+		     "google,gru-rev5", "google,gru-rev4",
+		     "google,gru-rev3", "google,gru-rev2",
+		     "google,gru", "rockchip,rk3399";
+
 - Google Jaq (Haier Chromebook 11 and more):
     Required root node properties:
       - compatible = "google,veyron-jaq-rev5", "google,veyron-jaq-rev4",
@@ -70,6 +81,15 @@ Rockchip platforms device tree bindings
 		     "google,veyron-jerry-rev3", "google,veyron-jerry",
 		     "google,veyron", "rockchip,rk3288";
 
+- Google Kevin (Samsung Chromebook Plus):
+    Required root node properties:
+      - compatible = "google,kevin-rev15", "google,kevin-rev14",
+		     "google,kevin-rev13", "google,kevin-rev12",
+		     "google,kevin-rev11", "google,kevin-rev10",
+		     "google,kevin-rev9", "google,kevin-rev8",
+		     "google,kevin-rev7", "google,kevin-rev6",
+		     "google,kevin", "google,gru", "rockchip,rk3399";
+
 - Google Mickey (Asus Chromebit CS10):
     Required root node properties:
       - compatible = "google,veyron-mickey-rev8", "google,veyron-mickey-rev7",