ab8500-mfd: add regulator support to ab8500 mfd device

Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Acked-By: Mattias Wallin <mattias.wallin@stericsson.com>
Acked-By: Bengt JONSSON <bengt.g.jonsson@stericsson.com>
Signed-off-by: Sundar R Iyer <sundar.iyer@stericsson.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>