regulator: db8500-prcmu: Separate regulator registration from probe

This will provide us with a convenient way to register regulators when
booting with Device Tree both enabled & disabled and will save us a
great deal of code duplication in time.

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