regulator: core: Limit regulators coupling to a single couple

Device tree binding was changed in a way that now max-spread values must
be defied per regulator pair. Limit number of pairs in order to adapt to
the new binding without changing regulators code.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>