regulators/db8500: split off shared dbx500 code

As we progress with DB5500 and future voltage domain regulators
based on very similar hardware as found in the DB8500 PRCMU,
it makes sense to split off the generic parts and introduce
some generic debug code for the DBx500 regulators. This patch
accoplish a basic abstraction of the DBx500 voltage domain
regulators.

Signed-off-by: Bengt Jonsson <bengt.g.jonsson@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>