regulator: da9211: Move buck modes into header file

This will allow device trees to make use of these constants.

Signed-off-by: Anand K Mistry <amistry@google.com>
Link: https://lore.kernel.org/r/20200702131350.1.I96e67ab7b4568287eb939e8a572cbc03e87f1aa0@changeid
Signed-off-by: Mark Brown <broonie@kernel.org>