regulator: extract voltage balancing code to the separate function

Move the coupled regulators voltage balancing code to the separate
function and allow to call it from the custom regulator couplers.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20200529124940.10675-2-m.szyprowski@samsung.com


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