regulator: Define full constraints function with REGULATOR disabled

This allows machine drivers to build without ifdefs if they have
full constraints. Suggested by machine drivers contributed by
Haojian Zhuang <haojian.zhuang@gmail.com>.

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