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'.

Change-Id: Ib3474908c50f8ad601e27a460a87e7b39869a768