dt-bindings: smb1398: Add binding documents for SMB1398 charger

Initial version of the binding document for SMB1398 charger, which
can work in DIV2 charge pump mode as downstream chargers or combo mode
as a pre-regulator.

Change-Id: If40a1166f05ae77d56b663ffc97546b7db7680bd
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>
Signed-off-by: Ashish Chavan <ashichav@codeaurora.org>