regulator: ab3100: refactor probe to use IDs

This refactors the AB3100 regulator probe to use regulator IDs
and pass this to a separate registration function. This works
much smoother when migrating to device tree, as we can use a
match table with this regulator ID encoded in the .driver_data.

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