regulator: core: Allow drivers to set voltage mapping table in regulator_desc

Some regulator hardware use table based mapping can set volt_table in
regulator_desc and use regulator_list_voltage_table() for their list_voltage
callback.

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