regulator: da9062: add of_map_mode support for bucks

This patch adds of_map_mode support for bucks to set regulator modes
from within regulator framework.

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Signed-off-by: Christian Hemp <c.hemp@phytec.de>
Signed-off-by: Stefan Riedmueller <s.riedmueller@phytec.de>
Link: https://lore.kernel.org/r/1573652416-9848-3-git-send-email-chf.fritz@googlemail.com
Signed-off-by: Mark Brown <broonie@kernel.org>