regulator: ab8500: provide per-regulator shift

This encodes the voltage shifts (the number of bits to shift
the register to get to the selector) into the regulator info.
We have several new variants of the AB8500 so this varies a
lot.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>