hwmon: (ad741x) Add DT bindings for Analog Devices AD741x

This adds device tree bindings for Analog Devices AD741x
as found in Gateway routers.

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