hwmon: (ltq-cputemp) add devicetree bindings documentation

Document the devicetree bindings for the ltq-cputemp
Signed-off-by: NFlorian Eckert <fe@dev.tdt.de>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/ltq-cputemp.txt

+Lantiq cpu temperatur sensor
+
+Requires node properties:
+- compatible value :
+	"lantiq,cputemp"
+
+Example:
+	cputemp@0 {
+		compatible = "lantiq,cputemp";
+	};