dt-bindings: power: supply: Add device-tree binding for Summit SMB3xx

Summit SMB3xx series is a Programmable Switching Li+ Battery Charger.
This patch adds device-tree binding for Summit SMB345, SMB347 and SMB358
chargers.

Signed-off-by: David Heidelberg <david@ixit.cz>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>