hwmon: (ina3221) Implement ti,single-shot DT property

By default, ina3221, as a hardware monitor, continuously measures
the inputs and generates corresponding data. However, for battery
powered devices, this mode might be power consuming.

The DT binding doc is updated with a new boolean type property to
allow changing the default operating mode from consuming mode to
single-shot mode, which will measure input on demand and then shut
down to save power.

So this patch implements the DT property accordingly.

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