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>