dt-bindings: hwmon: Add bindings for Winbond W83781D

This adds a device tree binding for the Winbond W83781D and its
sibling HW monitoring ICs. This is used in for example the Freecom
FSG-3 router/NAS.

Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210729230543.2853485-1-linus.walleij@linaro.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>