hwmon: (ina2xx) Add ina231 compatible string

Add support for "ina231" as compatible string, and update
Documentation and Kconfig accordingly.

Tested with the Exynos5422-based odroid-xu3 board which has on-board
INA231 sensors.

Signed-off-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
diff --git a/drivers/hwmon/Kconfig b/drivers/hwmon/Kconfig
index a7de26d..c243940 100644
--- a/drivers/hwmon/Kconfig
+++ b/drivers/hwmon/Kconfig
@@ -1430,8 +1430,8 @@
 	tristate "Texas Instruments INA219 and compatibles"
 	depends on I2C
 	help
-	  If you say yes here you get support for INA219, INA220, INA226, and
-	  INA230 power monitor chips.
+	  If you say yes here you get support for INA219, INA220, INA226,
+	  INA230, and INA231 power monitor chips.
 
 	  The INA2xx driver is configured for the default configuration of
 	  the part as described in the datasheet.