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>

Documentation/hwmon/ina3221.rst

@@ -41,6 +41,18 @@ curr[123]_max           Warning alert current(mA) setting, activates the
 			average is above this value.
 curr[123]_max_alarm     Warning alert current limit exceeded
 in[456]_input           Shunt voltage(uV) for channels 1, 2, and 3 respectively
+in7_input               Sum of shunt voltage(uV) channels
+in7_label               Channel label for sum of shunt voltage
+curr4_input             Sum of current(mA) measurement channels,
+                        (only available when all channels use the same resistor
+                        value for their shunt resistors)
+curr4_crit              Critical alert current(mA) setting for sum of current
+                        measurements, activates the corresponding alarm
+                        when the respective current is above this value
+                        (only effective when all channels use the same resistor
+                        value for their shunt resistors)
+curr4_crit_alarm        Critical alert current limit exceeded for sum of
+                        current measurements.
 samples                 Number of samples using in the averaging mode.
 
                         Supports the list of number of samples: