regulator: da9121: Add device variants

Add basic support for configuration to reference variants of this device,
and track the selected variant within the driver.

Signed-off-by: Adam Ward <Adam.Ward.opensource@diasemi.com>
Link: https://lore.kernel.org/r/aaabd3063593e5172fa6b605884d475df64e6d65.1606755367.git.Adam.Ward.opensource@diasemi.com
Signed-off-by: Mark Brown <broonie@kernel.org>