regulator: Convert mc13xxx regulator drivers to set_voltage_sel

Convert mc13892_sw_regulator_ops and mc13xxx_regulator_ops to set_voltage_sel.
mc13xxx_get_best_voltage_index function is not used now, remove it.

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