regulator: ab8500: Prepare the driver for additional platforms

More platforms are to be supported by the AB8500 regulator driver,
so in this patch we prepare for their arrival. Instead of using
the AB8500 settings blindly, we provide an infrastructure where
adding a new platform is as easy as providing a couple of platform
specific data structures.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>