regulator: helpers: Export helper voltage listing

Some drivers need to translate voltage values to selectors prior regulator
registration. Currently a regulator_desc based list_voltages helper is only
exported for regulators using the linear_ranges. Export similar helper also
for regulators using simple linear mapping.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Link: https://lore.kernel.org/r/1200ef7a50c84327ada019b85f6527b4fc9b5ce1.1617020713.git.matti.vaittinen@fi.rohmeurope.com
Signed-off-by: Mark Brown <broonie@kernel.org>