regulator: core: Remove regulator_list

As we are already registering a device with regulator_class for each
regulator device, regulator_list is redundant and can be replaced with
calls to class_find_device() and class_for_each_device().
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Mark Brown <broonie@kernel.org>