regulator: Don't disable unused regulators we don't have permission for

In the spirit of conservatism that governs our general approach to
permissions it is better if we don't touch regulators we weren't explicitly
given permissions to control. This avoids the need to explicitly specify
unknown regulators in DT as always on, if a regulator is not otherwise
involved in software control it can be omitted from the DT.

Regulators explicitly given constraints in DT still need to have an always
on constraint specified as before.

Signed-off-by: Mark Brown <broonie@linaro.org>