hwmon: (max6650) Add devicetree support documentation

This adds the devicetree binding documentation for the max6650 fan
controller.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

diff --git a/Documentation/devicetree/bindings/hwmon/max6650.txt b/Documentation/devicetree/bindings/hwmon/max6650.txt
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+Bindings for MAX6651 and MAX6650 I2C fan controllers
+
+Reference:
+[1]    https://datasheets.maximintegrated.com/en/ds/MAX6650-MAX6651.pdf
+
+Required properties:
+- compatible : One of "maxim,max6650" or "maxim,max6651"
+- reg        : I2C address, one of 0x1b, 0x1f, 0x4b, 0x48.
+
+Optional properties, default is to retain the chip's current setting:
+- maxim,fan-microvolt : The supply voltage of the fan, either 5000000 uV or
+                       12000000 uV.
+- maxim,fan-prescale  : Pre-scaling value, as per datasheet [1]. Lower values
+                       allow more fine-grained control of slower fans.
+                       Valid: 1, 2, 4, 8, 16.
+
+Example:
+       fan-max6650: max6650@1b {
+               reg = <0x1b>;
+               compatible = "maxim,max6650";
+               maxim,fan-microvolt = <12000000>;
+               maxim,fan-prescale = <4>;
+       };