dt-bindings: w1: document generic onewire bindings

This patch adds a generic w1 bindings document that describes how w1
slave deviceses are grouped under master nodes. It also augments the
existing w1-gpio.txt document a bit.

Signed-off-by: Daniel Mack <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Sebastian Reichel <[e-mail]>

Documentation/devicetree/bindings/w1/w1-gpio.txt

···
  - linux,open-drain: if specified, the data pin is considered in
 		     open-drain mode.
 
+Also refer to the generic w1.txt document.
+
 Examples:
 
-	onewire@0 {
+	onewire {
 		compatible = "w1-gpio";
-		gpios = <&gpio 126 0>, <&gpio 105 0>;
-	};
+		gpios = <&gpio 0 GPIO_ACTIVE_HIGH>;
 
+		battery {
+			// ...
+		};
+	};

Documentation/devicetree/bindings/w1/w1.txt

···
+Generic devicetree bindings for onewire (w1) busses
+===================================================
+
+Onewire busses are described through nodes of their master bus controller.
+Slave devices are listed as sub-nodes of such master devices. For now, only
+one slave is allowed per bus master.
+
+
+Example:
+
+	charger: charger {
+		compatible = "gpio-charger";
+		charger-type = "mains";
+		gpios = <&gpio 1 GPIO_ACTIVE_LOW>;
+	};
+
+	onewire {
+		compatible = "w1-gpio";
+		gpios = <&gpio 100 0>, <&gpio 101 0>;
+
+		battery {
+			compatible = "maxim,ds2760";
+			power-supplies = <&charger>;
+		};
+	};