Simplify the code and create hwmon attributes as well as hwmon device in one go.

With the new hwmon API, hwmon attributes are now attached to the hwmon device.
Therefore, split hwmon and device attributes into two separate groups.
Platform attributes are still attached to the platform device.

Also use devm_kzalloc to allocate local data structures for further
simplification.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>