power: battery: Add support for FCC stepping

On some PMIC designs a sudden increase or decrease in FCC
can impact the PMIC die thermally, causing the XO to drift.
Introduce a logic which gradually ramps up/down the FCC
whenever there is a change in its value, this spreads out
the thermals over-time. The default step rate is 100mA/sec
and the feature can be enabled using the DT property
"qcom,fcc-stepping-enable".

Change-Id: Ic89ffd3ea61f35bc09a8db3e8563c8b5b70d8bd2
Signed-off-by: Umang Agrawal <uagrawal@codeaurora.org>