Documentation: batterydata: Add avg vbat step charging support

Add support to specify average battery votlage based step charging
using qcom,vbat-avg-based-step-chg property. While at it, add
support to define whether to change the FCC directly or in steps
of pre-configured value using the property qcom,taper-fcc.

Change-Id: Icdbe5dd68bc7f8e2cb89aaf91b67b8f76891ace2
Signed-off-by: Umang Agrawal <uagrawal@codeaurora.org>