ARM: ux300: Fix unimplementable regulation constraints

It doesn't make sense to grant permission to change the status of a
regulator that is also set as always on and similarly it doesn't make
sense to allow a driver to change the voltage of a regulator which can
only be set to a single voltage.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>