bindings: smb1398: Add binding documents for SMB1398 charger

Add binding documentation for SMB1398 charger, which can work
in DIV2 charge pump mode as downstream chargers or combo mode
as a pre-regulator.

This is a snapshot from msm-4.19 commit 83784b9c3513
("dt-bindings: Add property to keep display reset gpio pin
high").

Change-Id: I96d552224f6384c68fce1398e14159c46c257210