regulator: Properly register dummy regulator driver

Recent changes in the driver core appear to mean that the data structures
for the driver core are not fully initialised unless the driver is bound.
Make sure the driver core knows the dummy driver is in use by binding it
to a driver.

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