asmc: introduce the concept of generic models

Having to enter in each of the models for Apple hardware, recompiling,
etc, is tedious. Provide generic models so end-users can leverage some
of the capabilities provided by the driver, i.e., common features like
minimal fans and lights (if present on the generic model) support.

The generic models are as follows:

• Macmini
• MacBookAir
• MacBookPro
• MacPro

This sort of follows the pattern established by the applesmc driver in
Linux.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D55395