hwmon: (ina3221) Add voltage conversion time settings

The CONFIG register has two 3-bit fields for conversion time
settings of Bus-voltage and Shunt-voltage, respectively. The
conversion settings, along with averaging mode, allow users
to optimize available timing requirement.

This patch adds an 'update_interval' sysfs node through the
hwmon_chip_info of hwmon core. It reflects a total hardware
conversion time:
    samples * channels * (Bus + Shunt conversion times)

Though INA3221 supports different conversion time setups for
Bus and Shunt voltages, this patch only adds the support of
a unified setting for both conversion times, by dividing the
conversion time into two equal values.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
[groeck: .rst related formatting changes in documentation]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>