regulator: switch the ab3100 to use enable_time()

Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

This switches the ab3100 core to use the enable_time()
infrastructure from the regulator core.

Cc: Mattias Wallin <mattias.wallin@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>