regulator: Factor out voltage constraint setup

This allows constraints to take effect on regulators that support
voltage setting but for which the board does not specify a voltage
range (for example, because it is fixed correctly at system startup).

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>