regulator: 88pm8607: Simplify pm8607_list_voltage implementation

Set volt_table filed then we can use regulator_list_voltage_table.
Since we have volt_table setting, now we can remove vol_table from
struct pm8607_regulator_info.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>