regulator: Add missing n_voltages setting for 88pm8607

Then we can remove the vol_nbits field from struct pm8607_regulator_info.

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>