Document valid mode values for BUCK regulators.

Signed-off-by: Anand K Mistry <amistry@google.com>
Link: https://lore.kernel.org/r/20200702162231.v2.2.I0a814b246cfe47f8dd1e25553ee881cbcfd0d8eb@changeid


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