dt-bindings: hwmon: Document adt7475 pwm-active-state property

Add binding information for the adi,pwm-active-state property.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>