regulator: add unset_regulator_supplies to fix regulator_unregister

Currently regulator_unregister does not clear regulator <--> consumer
mapping.
This patch introduces unset_regulator_supplies that clear the map.

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