power: qpnp-fg: add support for capacity learning feedback

Capacity learning feedback is needed for the capacity learning
algorithm. It will be supported based on the following device
tree property "qcom,capacity-learning-feedback" and will work
only when the capacity learning is enabled.

CRs-Fixed: 848838
Change-Id: Idde0d3aa6e8a3aafeadf5f90e2118ceed2c96b5f
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>