regulator: core: Use the power efficient workqueue for delayed powerdown
Adapted for 3.4 from lsk-v3.10

There is no need to use a normal per-CPU workqueue for delayed power downs
as they're not timing or performance critical and waking up a core for them
would defeat some of the point.

Signed-off-by: Mark Brown <broonie@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Liam Girdwood <liam.r.girdwood@intel.com>
(cherry picked from commit 070260f)
Signed-off-by: Luis Cruz <ljc2491@gmail.com>