dt-bindings: mfd: da9062: describe buck modes

This patch adds DT description of da9062 buck regulator modes.

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Link: https://lore.kernel.org/r/1573652416-9848-4-git-send-email-chf.fritz@googlemail.com


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