hwmon: (ina3221) Add summation feature support

This patch implements the summation feature of INA3221, mainly the
SCC (enabling) and SF (warning flag) bits of MASK_ENABLE register,
INA3221_SHUNT_SUM (summation of shunt voltages) register, and the
INA3221_CRIT_SUM (its critical alert setting) register.

Although the summation feature allows user to select which channels
to be added to the result, as an initial support, this patch simply
selects all channels by default, with one only condition: all shunt
resistor values need to be the same. This is because the summation
of current channels can be only accurately calculated, using shunt
voltage sum register, if all shunt resistors are equivalent.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Link: https://lore.kernel.org/r/20191016235702.22039-1-nicoleotsuka@gmail.com
[groeck: summation->sum in documentation and label]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>