hwmon: (ina2xx) Document compatible for INA231

Document the compatible for INA231 sensor.
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/ina2xx.txt

@@ -7,6 +7,7 @@ Required properties:
 	- "ti,ina220" for ina220
 	- "ti,ina226" for ina226
 	- "ti,ina230" for ina230
+	- "ti,ina231" for ina231
 - reg: I2C address
 
 Optional properties: