hwmon: (pmbus/max31785) Add dual tachometer support

The dual tachometer feature is implemented in hardware with a TACHSEL
input to indicate the rotor under measurement, and exposed on the device
by extending the READ_FAN_SPEED_1 word with two extra bytes*. The need
to read the non-standard four-byte response leads to a cut-down
implementation of i2c_smbus_xfer_emulated() included in the driver.
Further, to expose the second rotor tachometer value to userspace the
values are exposed through virtual pages. We re-route accesses to
FAN_CONFIG_1_2 and READ_FAN_SPEED_1 on pages 23-28 (not defined by the
hardware) to the same registers on pages 0-5, and with the latter command
we extract the value from the second word of the four-byte response.

* The documentation recommends the slower rotor be associated with
TACHSEL=0, which corresponds to the first word of the response. The
TACHSEL=0 measurement is used by the controller's closed-loop fan
management to judge target fan rate.

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

Documentation/hwmon/max31785

@@ -17,8 +17,9 @@ management with temperature and remote voltage sensing. Various fan control
 features are provided, including PWM frequency control, temperature hysteresis,
 dual tachometer measurements, and fan health monitoring.
 
-For dual rotor fan configuration, the MAX31785 exposes the slowest rotor of the
-two in the fan[1-4]_input attributes.
+For dual-rotor configurations the MAX31785A exposes the second rotor tachometer
+readings in attributes fan[5-8]_input. By contrast the MAX31785 only exposes
+the slowest rotor measurement, and does so in the fan[1-4]_input attributes.
 
 Usage Notes
 -----------
@@ -31,7 +32,8 @@ Sysfs attributes
 
 fan[1-4]_alarm		Fan alarm.
 fan[1-4]_fault		Fan fault.
-fan[1-4]_input		Fan RPM.
+fan[1-8]_input		Fan RPM. On the MAX31785A, inputs 5-8 correspond to the
+			second rotor of fans 1-4
 fan[1-4]_target		Fan input target
 
 in[1-6]_crit		Critical maximum output voltage