hwmon: Add Nuvoton NCT7904 hwmon driver

The NCT7904D is a hardware monitor supporting up to 20 voltage sensors,
internal temperature sensor, Intel PECI and AMD SB-TSI CPU temperature
interface, up to 12 fan tachometer inputs, up to 4 fan control channels
with SmartFan.

Signed-off-by: Vadim V. Vlasov <vvlasov@dev.rtsoft.ru>
[Guenter Roeck: Fixed whitespace errors, dropped redundant comment]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/nct7904

@@ -0,0 +1,60 @@
+Kernel driver nct7904
+====================
+
+Supported chip:
+  * Nuvoton NCT7904D
+    Prefix: nct7904
+    Addresses: I2C 0x2d, 0x2e
+    Datasheet: Publicly available at Nuvoton website
+	http://www.nuvoton.com/
+
+Author: Vadim V. Vlasov <vvlasov@dev.rtsoft.ru>
+
+
+Description
+-----------
+
+The NCT7904D is a hardware monitor supporting up to 20 voltage sensors,
+internal temperature sensor, Intel PECI and AMD SB-TSI CPU temperature
+interface, up to 12 fan tachometer inputs, up to 4 fan control channels
+with SmartFan.
+
+
+Sysfs entries
+-------------
+
+Currently, the driver supports only the following features:
+
+in[1-20]_input		Input voltage measurements (mV)
+
+fan[1-12]_input		Fan tachometer measurements (rpm)
+
+temp1_input		Local temperature (1/1000 degree,
+			0.125 degree resolution)
+
+temp[2-9]_input		CPU temperatures (1/1000 degree,
+			0.125 degree resolution)
+
+fan[1-4]_mode		R/W, 0/1 for manual or SmartFan mode
+			Setting SmartFan mode is supported only if it has been
+			previously configured by BIOS (or configuration EEPROM)
+
+fan[1-4]_pwm		R/O in SmartFan mode, R/W in manual control mode
+
+The driver checks sensor control registers and does not export the sensors
+that are not enabled. Anyway, a sensor that is enabled may actually be not
+connected and thus provide zero readings.
+
+
+Limitations
+-----------
+
+The following features are not supported in current version:
+
+ - SmartFan control
+ - Watchdog
+ - GPIO
+ - external temperature sensors
+ - SMI
+ - min/max values
+ - many other...