regulator: twl: split twl6030 logic into its own file

In order to not break existing users, we keep using the same
CONFIG symbol.
This makes it easier to add support for TWL6032 and refactor
mfd/twl-core.
Checkpatch warnings are inherited from twl-regulator.c and will
be addressed in a subsequent patch.

Signed-off-by: Nicolae Rosia <Nicolae_Rosia@mentor.com>
Signed-off-by: Mark Brown <broonie@kernel.org>