Regulator: Push lock out of _notifier_call_chain + add voltage change event.

Regulator: Push lock out of _notifier_call_chain and into caller functions
(side effect of fixing deadlock in regulator_force_disable)
+ Add a voltage changed event.
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>