bindings: SMB358: Add documentation for smb358 charger

Add snapshot of smb358 charger documentation from msm-4.14
'commit 50f9f83dd79b ("Merge "defconfig : Enable pinctrl and
clock driver for SA8195"")'.

SMB358 is a single-cell battery charger. It can charge
the battery and power the system via the USB/AC adapter input.

Change-Id: Id3aef2dab81a67061d728f661ce568917e86b08e