regulator: Support set_voltage_time_sel for drivers implement set_voltage

In currently implementation of _regulator_do_set_voltage, set_voltage_time_sel will
only be called if set_voltage_sel is implemented.

set_voltage_time_sel actually only needs get_voltage_sel to get old_selector.

This patch makes regulator core support set_voltage_time_sel for drivers
implement either set_voltage or set_voltage_sel.

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