hwmon: (ina3221) Add averaging mode support

The CONFIG register has a 3-bit averaging mode field for users
to setup the number of samples that are collected and averaged
together. This is very useful to filter noise from sensor data.

This patch adds a 'samples' sysfs node using hwmon_chip_samples
of hwmon core, and updates wait time calculation by taking this
samples value into account.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>