regulator: da9121: add current support

This commit adds support for getting/setting current for all supported
variants. Limits are adjusted per variant to match HW implementation.

Signed-off-by: Adam Ward <Adam.Ward.opensource@diasemi.com>
Link: https://lore.kernel.org/r/9aa80b909893dbe609730919ed595c6a8ac26606.1606755367.git.Adam.Ward.opensource@diasemi.com


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