regulator: core: Use ww_mutex for regulators locking

Wait/wound mutex shall be used in order to avoid lockups on locking of
coupled regulators.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Suggested-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: digetx