hwmon: (w83791d) add pwm_enable support

Add support for pwm_enable.

Signed-off-by: Marc Hulsman <m.hulsman@tudelft.nl>
Acked-by: Hans de Goede <j.w.r.degoede@hhs.nl>
Signed-off-by: Jean Delvare <khali@linux-fr.org>

Documentation/hwmon/w83791d

@@ -108,6 +108,17 @@ going forward.
 The driver reads the hardware chip values at most once every three seconds.
 User mode code requesting values more often will receive cached values.
 
+/sys files
+----------
+The sysfs-interface is documented in the 'sysfs-interface' file. Only
+chip-specific options are documented here.
+
+pwm[1-3]_enable -	this file controls mode of fan/temperature control for
+			fan 1-3. Fan/PWM 4-5 only support manual mode.
+		            * 1 Manual mode
+		            * 2 Thermal Cruise mode   (no further support)
+		            * 3 Fan Speed Cruise mode (no further support)
+
 Alarms bitmap vs. beep_mask bitmask
 ------------------------------------
 For legacy code using the alarms and beep_mask files:
@@ -138,4 +149,4 @@ global_enable:  alarms: -------- beep_mask: 0x800000 (modified via beep_enable)
 
 W83791D TODO:
 ---------------
-Provide a patch for smart-fan control (still need appropriate motherboard/fans)
+Provide a patch for Thermal Cruise registers.