hwmon: (pmbus/max31785) Add fan control

The implementation makes use of the new fan control virtual registers
exposed by the pmbus core. It mixes use of the default implementations
with some overrides via the read/write handlers to handle FAN_COMMAND_1
on the MAX31785, whose definition breaks the value range into various
control bands dependent on RPM or PWM mode.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/max31785

@@ -32,6 +32,7 @@ Sysfs attributes
 fan[1-4]_alarm		Fan alarm.
 fan[1-4]_fault		Fan fault.
 fan[1-4]_input		Fan RPM.
+fan[1-4]_target		Fan input target
 
 in[1-6]_crit		Critical maximum output voltage
 in[1-6]_crit_alarm	Output voltage critical high alarm
@@ -44,6 +45,12 @@ in[1-6]_max_alarm	Output voltage high alarm
 in[1-6]_min		Minimum output voltage
 in[1-6]_min_alarm	Output voltage low alarm
 
+pwm[1-4]		Fan target duty cycle (0..255)
+pwm[1-4]_enable		0: Full-speed
+			1: Manual PWM control
+			2: Automatic PWM (tach-feedback RPM fan-control)
+			3: Automatic closed-loop (temp-feedback fan-control)
+
 temp[1-11]_crit		Critical high temperature
 temp[1-11]_crit_alarm	Chip temperature critical high alarm
 temp[1-11]_input	Measured temperature