regulator: mt6397: 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/20200702162231.v2.1.Icf69e2041b1af4548347018186c3ba6310f53e66@changeid


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