regulator: twl-regulator: rework fixed regulator definition

TWL603X and TWL4030 are different and have different code logic.
Rework the regulator definition method so we can split the file
easily in twl4030 and twl6030.

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