regulator: rohm-regulator: add helper for restricted voltage setting

Few ROHM PMICs have regulators where voltage setting can be done only
when regulator is disabled. Add helper for those PMICs.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Link: https://lore.kernel.org/r/6f51871e9fea611d133b5dd2560f4a7ee1ede9cd.1637233864.git.matti.vaittinen@fi.rohmeurope.com


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