Driver core: Add support for compatibility classes

When turning class devices into bus devices, we may need to
temporarily add links in sysfs so that user-space applications
are not confused. This is done by adding the following API:

* Functions to register and unregister compatibility classes.
  These appear in sysfs at the same location as regular classes, but
  instead of class devices, they contain links to bus devices.
* Functions to create and delete such links. Additionally, the caller
  can optionally pass a target device to which a "device" link should
  point (typically that would be the device's parent), to fully emulate
  the original class device.

The i2c subsystem will be the first user of this API, as i2c adapters
are being converted from class devices to bus devices.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>