regulator: ab8500: Update voltage handling for fixed voltage regulators

There are a few over-lapping methods for voltage selection operating
in the AB8500 regulator driver currently. This patch removes unused,
unnecessary variables from the regulator_info structures and provides
voltage tables for those regulators which have fixed voltages.
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>