hwmon: add support for GMT G760A fan speed PWM controller

This controller can be found on the D-Link DNS-323 for instance, where
it is to be configured via static i2c_board_info in the board-specific
mach-orion/dns323-setup.c; this driver supports only the new-style
driver model.

Tested-by: Martin Michlmayr <tbm@cyrius.com>
Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Signed-off-by: Matthew Palmer <mpalmer@debian.org>
Signed-off-by: Laurie Bradshaw <bradshaw.laurie@googlemail.com>
Signed-off-by: Martin Michlmayr <tbm@cyrius.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/hwmon/g760a

@@ -0,0 +1,36 @@
+Kernel driver g760a
+===================
+
+Supported chips:
+  * Global Mixed-mode Technology Inc. G760A
+    Prefix: 'g760a'
+    Datasheet: Publicly available at the GMT website
+      http://www.gmt.com.tw/datasheet/g760a.pdf
+
+Author: Herbert Valerio Riedel <hvr@gnu.org>
+
+Description
+-----------
+
+The GMT G760A Fan Speed PWM Controller is connected directly to a fan
+and performs closed-loop control of the fan speed.
+
+The fan speed is programmed by setting the period via 'pwm1' of two
+consecutive speed pulses. The period is defined in terms of clock
+cycle counts of an assumed 32kHz clock source.
+
+Setting a period of 0 stops the fan; setting the period to 255 sets
+fan to maximum speed.
+
+The measured fan rotation speed returned via 'fan1_input' is derived
+from the measured speed pulse period by assuming again a 32kHz clock
+source and a 2 pulse-per-revolution fan.
+
+The 'alarms' file provides access to the two alarm bits provided by
+the G760A chip's status register: Bit 0 is set when the actual fan
+speed differs more than 20% with respect to the programmed fan speed;
+bit 1 is set when fan speed is below 1920 RPM.
+
+The g760a driver will not update its values more frequently than every
+other second; reading them more often will do no harm, but will return
+'old' values.