devicetree: hwmon: Add documentation for TMP108 driver.

Simple hwmon binding documentation.
Signed-off-by: John Muir <john@jmuir.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/tmp108.txt

+TMP108 temperature sensor
+-------------------------
+
+This device supports I2C only.
+
+Requires node properties:
+- compatible : "ti,tmp108"
+- reg : the I2C address of the device. This is 0x48, 0x49, 0x4a, or 0x4b.
+
+Example:
+	tmp108@48 {
+		compatible = "ti,tmp108";
+		reg = <0x48>;
+	};