dt-bindings: hwmon: Add AXI FAN Control documentation

Document the AXI FAN Control IP core devicetree bindings.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191009102806.262241-2-nuno.sa@analog.com


[groeck: adi,pulses-per-revolution -> pulses-per-revolution]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>