regulator: Make regulator_sync_voltage() usable by coupled regulators

regulator: Make regulator_sync_voltage() usable by coupled regulators

Make regulator_sync_voltage() to re-balance voltage state of a coupled
regulators instead of changing the voltage directly.

Tested-by: Peter Geis <pgwipeout@gmail.com> # Ouya T30
Tested-by: Dmitry Osipenko <digetx@gmail.com> # A500 T20 and Nexus7 T30
Tested-by: Nicolas Chauvet <kwizart@gmail.com> # PAZ00 T20
Tested-by: Matt Merhar <mattmerhar@protonmail.com> # Ouya T30
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20210122174311.28230-1-digetx@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>