power: qpnp-fg: add support for battery age estimation

Add support to estimate the usable capacity of the battery from various
fuel gauge inputs. Provide two algorithms, one to estimate the battery
capacity with the measured battery ESR, and another to learn the battery
capacity by doing software coulomb counting.

CRs-Fixed: 771998
Change-Id: Ie44a24549eb6e7628eb86f1bfb084c1e35b2067a
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>