regulator: da9121: Add device variant descriptors

Descriptors for bucks in all variants, ready for of_regulator_match

Signed-off-by: Adam Ward <Adam.Ward.opensource@diasemi.com>
Link: https://lore.kernel.org/r/8f7a11d59e5ac3ba1bfd448bcfc905efc99b7874.1606755367.git.Adam.Ward.opensource@diasemi.com


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