regulator: da9121: Add device variant regmaps

Add ability to probe device and validate configuration, then apply a regmap
configuration for a single or dual buck device accordingly.

Signed-off-by: Adam Ward <Adam.Ward.opensource@diasemi.com>
Link: https://lore.kernel.org/r/068c6b8d5e1b4e221e899e4c914c429429a2ec7d.1606755367.git.Adam.Ward.opensource@diasemi.com


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