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>