dt-bindings: hwmon: Document adt7475 bypass-attenuator property

Add documentation for the bypass-attenuator-in[0-4] property.

Signed-off-by: Logan Shaw <logan.shaw@alliedtelesis.co.nz>
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>