regulator: add notifier event for regulator enable

Add a notifier event which is triggered when a regulator is
enabled.  This can be used by a driver which must configure the
hardware state whenever a given supply is enabled even if some
other consumer was responsible for enabling the supply.

Change-Id: Ic8363308b64953451dd01756e1bfdc2a64527096
Signed-off-by: David Collins <collinsd@codeaurora.org>