regulator: core: Refactor regulator_list_voltage()

Change _regulator_list_voltage() argument from regulator to
regulator_dev in order to provide better separation of core layers.
Allow calling _regulator_list_voltage() from functions, with
regulator_dev argument. This refactoring is needed in order to
implement setting voltage of coupled regulators.

Signed-off-by: Maciej Purski <m.purski@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>