regulator: Add missing n_voltages setting for max8925

The n_voltages are the same for all regulators: (max - min / step) + 1 == 64.
The vol_shift is always 0, and the vol_nbits is always the same as n_voltages
setting. Thus we can remove vol_shitf and vol_nbits fields from struct
max8925_regulator_info.

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