ASoC: Add a notifier for jack status changes

Some systems provide both mechanical and electrical detection of jack
status changes. On such systems power savings can be achieved by only
enabling the electrical detection methods when physical insertion has
been detected.

Begin supporting such systems by providing a notifier for jack status
changes which can be used to trigger any reconfiguration.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>