regulator: twl4030 voltage enumeration (v2)

Update previously-posted twl4030 regulator driver to export
supported voltages to upper layers using a new mechanism.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>