ccwgroup bus driver.

The ccwgroup driver contains 'ccw_group' devices, which needed because of
strange hardware properties: Some device drivers need two or more
ccw_devices to make up a logical device and the user has to configure
which ones belong together. We have not found a way to represent this
correctly using the linux driver model, the current implementation
is about as good as it gets.

Authors: Arnd Bergmann <arndb@de.ibm.com>,