regulator: da9121: Update registration to support multiple buck variants

Add function which iterates the regulator descriptors for the confirmed
variant ID and registers each buck.

Signed-off-by: Adam Ward <Adam.Ward.opensource@diasemi.com>
Link: https://lore.kernel.org/r/1fd53c3ab032ef3c8a92f80a2247381db1c09ced.1606755367.git.Adam.Ward.opensource@diasemi.com


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