regulator: Build on non-ARM platforms

When the regulator API was merged it was added to the separate Kconfig
which ARM uses for drivers but not the generic one in drivers/.  Since
there is nothing ARM-specific about the API add it there too.

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