Device classes describe a type (or class) of device, like an input device
or network device, etc. This changeset defines a struct device_class that 
each subsystem is expected to implement and register with the core.

struct device_driver gains a devclass pointer which points to the class it
belongs to. When the driver is registered, it is added to the class's list
of drivers. Whenever a device is bound to that driver, it is added to the
class by calling the class's add_device callback. 

struct device gains a class_num field which is the per-class enumerated value
of the device. It is incremented each time a device is registered with the
class.

Each device class gets a driverfs directory in class/<class name> and two
subdirectories: 'devices' and 'drivers'. For each device added to the class,
a symlink is created in the devices/ directory that points to the device's
directory in the physical hierarchy. The name of the symlink is the enumerated
number the device got when it was registered with the class.

For each driver that's added to the class, a symlink is created in the class's
drivers/ directory that points to the driver's directory. The name of this 
symlink is a concatenation of <bus name>:<driver name> (to prevent namespace
conflicts of drivers with the same name on different buses).