hwmon: (ina2xx) implement update_interval attribute for ina226

This attribute allows to configure the update interval of ina226. Although
the bus and shunt voltage conversion times remain hardcoded to 1.1 ms, we can
now modify said interval by changing the averaging rate.

While we're at it - add an additional variable to ina2xx_data, which holds
the current configuration settings - this way we'll be able to restore the
configuration in case of an unexpected chip reset.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/ina2xx

@@ -48,3 +48,10 @@ The shunt value in micro-ohms can be set via platform data or device tree at
 compile-time or via the shunt_resistor attribute in sysfs at run-time. Please
 refer to the Documentation/devicetree/bindings/i2c/ina2xx.txt for bindings
 if the device tree is used.
+
+Additionally ina226 supports update_interval attribute as described in
+Documentation/hwmon/sysfs-interface. Internally the interval is the sum of
+bus and shunt voltage conversion times multiplied by the averaging rate. We
+don't touch the conversion times and only modify the number of averages. The
+lower limit of the update_interval is 2 ms, the upper limit is 2253 ms.
+The actual programmed interval may vary from the desired value.