regulator: core: Allow generic coupling only for always-on regulators

The generic voltage balancer doesn't work correctly if one of regulator
couples turns off. Currently there are no users in kernel for that case,
although let's explicitly show that this case is unsupported for those who
will try to use that feature.

Link: https://lore.kernel.org/linux-samsung-soc/20191008170503.yd6GscYPLxjgrXqDuCO7AJc6i6egNZGJkVWHLlCxvA4@z/


Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20191025002240.25288-2-digetx@gmail.com


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