regulator: core: Add new notification for enabling of regulator

This is useful for devices, which need some time to start up, to help
the drivers track how long the supply has been up already. Ie whether
it can safely talk to the HW or needs to wait.

Signed-off-by: Harald Geyer <harald@ccbib.org>
Signed-off-by: Mark Brown <broonie@kernel.org>