hwmon: (core) Rename groups parameter in API to extra_groups

The 'groups' parameter of hwmon_device_register_with_info() and
devm_hwmon_device_register_with_info() is only necessary if extra
non-standard attributes need to be provided. Rename the parameter
to extra_groups and clarify the documentation.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>