regulator: fixed: support using power domain for enable/disable

Adds possibility to choose the compatible "fixed-regulator-domain" for
regulators which use power domain for enabling/disabling corresponding
regulator.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20201023131925.334864-3-dmitry.baryshkov@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>