regulator: ab8500: Get rid of voltage_shift field from struct ab8500_regulator_info

The voltage_shift can be calculated from voltage_mask.
Let's remove voltage_shift fied from struct ab8500_regulator_info, this change
can prevent missing voltage_shift setting issue.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Bengt Jonsson <bengt.g.jonsson@stericsson.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>