regulator: machine driver interface

This interface is for machine specific code and allows the creation of
voltage/current domains (with constraints) for each regulator. It can
provide regulator constraints that will prevent device damage through
overvoltage or over current caused by buggy client drivers. It also
allows the creation of a regulator tree whereby some regulators are
supplied by others (similar to a clock tree).

Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>
Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>