regulator: Add option for machine drivers to enable the dummy regulator

Allow machine drivers to explicitly enable the use of the dummy regulator,
enabling simpler support for systems with only a few specific supplies
visible to software.

It is strongly recommended that this is not used on systems with
substantial software control over their PMICs, for maximum functionality
constrints should be as fully specified as possible.

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