regulator: add support for fixed regulators.

This adds supports for regulator that are not software controlable. It allows
them to coexist in systems with mixed supplies.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>