hwmon: (wm831x) Convert to devm_hwmon_device_register_with_groups

Use ATTRIBUTE_GROUPS macro and devm_hwmon_device_register_with_groups() to
simplify the code a bit.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>