regulator: Set list_voltage callback for 88pm8607

Current code implements pm8607_list_voltage but does not set the list_voltage
callback function in pm8607_regulator_ops. Fix it.

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