Regulators: virtual - use sysfs attribute groups

Instead of open-coding sysfs attribute group use canned solution.
Also add __devinit/__devexit markups for probe and remove methods
and use 'bool' where it makes sense.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>