regulator: Make fixed voltage regulators visible in Kconfig

This allows users to enable or disable support for these regulators at
build time as they can for other regulators rather than having platforms
force the regulators to be built in.

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