power: qpnp-fg: add support for IADC gain compensation for PMI8950

In the case of internal current sensing IADC gain has dependency
on VBAT during discharge. Add support for gain compensation to
negate the effect of VBAT.

CRs-Fixed: 859531
Change-Id: I518de6af3c8749f5b3dceef14af2d799ae6485ea
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>