regulator: scmi: Allow for zero voltage domains

SCMI Voltage protocol allows the platform to report no voltage domains
on discovery, while warning the user about such an odd configuration.
As a consequence this condition should not be treated as error by the SCMI
regulator driver either.

Allow SCMI regulator driver to probe successfully even when no voltage
domains are discovered.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20230126180511.766373-1-cristian.marussi@arm.com


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