regulator: Use list_voltage() to get voltage in regulator_set_voltage_time_sel

With this change, regulator_set_voltage_time_sel() can be more generic and not
limited to linear and table based mapping now.
One side-effect of this change is that list_voltage() must be implemented.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>