hwmon: (ad7418) Add device tree probing

This adds device tree probing for the AD7418 hwmon sensor.
When device tree is not enabled, stub functions will kick
in.

Tested on the Gateway Cambria GW2358-4.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[groeck: Added missing {} to terminate ad7418_dt_ids]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>