driver core: bus_type: add bus_groups

attribute groups are much more flexible than just a list of attributes,
due to their support for visibility of the attributes, and binary
attributes. Add bus_groups to struct bus_type which should be used
instead of bus_attrs.

bus_attrs will be removed from the structure soon.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>