auto_hotplug: use power efficient workqueue

Signed-off-by: revjunkie <gadgetjuan@gmail.com>
workqueue: Add system wide power_efficient workqueues Adapted for 3.4 from lsk-v3.10

This patch adds system wide workqueues aligned towards power saving. This is
done by allocating them with WQ_UNBOUND flag if 'wq_power_efficient' is set to
'true'.

Signed-off-by: revjunkie <gadgetjuan@gmail.com>