dock: introduce .uevent for devices in dock, eg libata

dock's uevent reported itself, not ata. It might be difficult to find an
ata device just according to a dock. This patch introduces docking ops
for each device in a dock. when docking, dock driver can send device
specific uevent. This should help dock station too (not just bay)

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Acked-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Len Brown <len.brown@intel.com>