regulator: da9121: Add support for device variants via devicetree

Add devicetree configuration and device variant parameters. Use the latter
to enable the check and use of parameters specific to dual buck variants.

Signed-off-by: Adam Ward <Adam.Ward.opensource@diasemi.com>
Link: https://lore.kernel.org/r/5849ce60595aef1018bdde7dcfb54a7397597545.1606755367.git.Adam.Ward.opensource@diasemi.com


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