dt-bindings: hwmon: Document adt7475 pin-function properties

dt-bindings: hwmon: Document adt7475 pin-function properties

The adt7473, adt7475, adt7476 and adt7490 have pins that can be used for
different functions. Add bindings so that it is possible to describe
what pin functions are intended by the hardware design.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20220323034056.260455-2-chris.packham@alliedtelesis.co.nz
Signed-off-by: Guenter Roeck <linux@roeck-us.net>