regulator: Add notifier event on regulator disable

regulator: Add notifier event on regulator disable

The intended use case is for drivers which disable regulators to save
power but need to do some work to restore the hardware state when
restarting.  If the supplies are not actually disabled due to board
limits or sharing with other active devices this notifier allows the
driver to avoid unneeded reinitialisation, particularly when used with
runtime PM.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>