hwmon: Create an NSA320 hardware monitoring driver

Create a driver to support the hardware monitoring chip present in
the Zyxel NSA320 and some of the other Zyxel NAS devices.

The driver reads fan speed and temperature from a suitably
pre-programmed MCU on the device.

Signed-off-by: Adam Baker <linux@baker-net.org.uk>
[groeck: Dropped .owner field initialization]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/nsa320

@@ -0,0 +1,53 @@
+Kernel driver nsa320_hwmon
+==========================
+
+Supported chips:
+  * Holtek HT46R065 microcontroller with onboard firmware that configures
+	it to act as a hardware monitor.
+    Prefix: 'nsa320'
+    Addresses scanned: none
+    Datasheet: Not available, driver was reverse engineered based upon the
+	Zyxel kernel source
+
+Author:
+  Adam Baker <linux@baker-net.org.uk>
+
+Description
+-----------
+
+This chip is known to be used in the Zyxel NSA320 and NSA325 NAS Units and
+also in some variants of the NSA310 but the driver has only been tested
+on the NSA320. In all of these devices it is connected to the same 3 GPIO
+lines which are used to provide chip select, clock and data lines. The
+interface behaves similarly to SPI but at much lower speeds than are normally
+used for SPI.
+
+Following each chip select pulse the chip will generate a single 32 bit word
+that contains 0x55 as a marker to indicate that data is being read correctly,
+followed by an 8 bit fan speed in 100s of RPM and a 16 bit temperature in
+tenths of a degree.
+
+
+sysfs-Interface
+---------------
+
+temp1_input - temperature input
+fan1_input - fan speed
+
+Notes
+-----
+
+The access timings used in the driver are the same as used in the Zyxel
+provided kernel. Testing has shown that if the delay between chip select and
+the first clock pulse is reduced from 100 ms to just under 10ms then the chip
+will not produce any output. If the duration of either phase of the clock
+is reduced from 100 us to less than 15 us then data pulses are likely to be
+read twice corrupting the output. The above analysis is based upon a sample
+of one unit but suggests that the Zyxel provided delay values include a
+reasonable tolerance.
+
+The driver incorporates a limit that it will not check for updated values
+faster than once a second. This is because the hardware takes a relatively long
+time to read the data from the device and when it does it reads both temp and
+fan speed. As the most likely case for two accesses in quick succession is
+to read both of these values avoiding a second read delay is desirable.