power: qpnp-fg: add support to control voltage gain dynamically

There is a requirement to control voltage gain settings when the
battery monotonic SOC is less than certain threshold and device
is discharging. New gain setting will be applied when the SOC
goes below the specified threshold and device is discharging.
Default gain setting will be restored otherwise.

This feature is controlled by "qcom-fg-dischg-voltage-gain-ctrl"
device tree property primarily. Other device tree parameters are
available to configure the SOC limit and the voltage gain to be
applied when the battery SOC goes below that limit.

CRs-Fixed: 977695
Change-Id: I2877f9e2b4c3aec72e0eee6d60b012f869523ed6
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>