driver core: Add the device driver-model structures to kerneldoc

Add the comments to the structure bus_type, device_driver, device,
class to device.h for generating the driver-model kerneldoc. With another patch
these all removed from the files in Documentation/driver-model/ since
they are out of date. That will keep things up to date and provide a better way
to document this stuff.

Signed-off-by: Wanlong Gao <wanlong.gao@gmail.com>
Acked-by: Harry Wei <harryxiyou@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>