hwmon: (nct6775) Add support for weighted fan control

The NCT677X series support weighted fan control. In this mode, a secondary
temperature source is used in addition to the primary temperature source to
control fan speed. Add support for this feature.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/nct6775

@@ -92,6 +92,25 @@ Common fan control attributes
 
 pwm[1-5]_temp_sel	Temperature source. Value is temperature sensor index.
 			For example, select '1' for temp1_input.
+pwm[1-5]_weight_temp_sel
+			Secondary temperature source. Value is temperature
+			sensor index. For example, select '1' for temp1_input.
+			Set to 0 to disable secondary temperature control.
+
+If secondary temperature functionality is enabled, it is controlled with the
+following attributes.
+
+pwm[1-5]_weight_duty_step
+			Duty step size.
+pwm[1-5]_weight_temp_step
+			Temperature step size. With each step over
+			temp_step_base, the value of weight_duty_step is added
+			to the current pwm value.
+pwm[1-5]_weight_temp_step_base
+			Temperature at which secondary temperature control kicks
+			in.
+pwm[1-5]_weight_temp_step_tol
+			Temperature step tolerance.
 
 Thermal Cruise mode (2)
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