power: qpnp-qg: Add the filtered voltage based SOC scaling (FVSS)

FVSS scales the SOC to 0 at the programmed cutoff
voltage. It uses the filtered VBAT (fifo-average)
to scale the SOC. FVSS uses a weighted average of
VBAT based SOC and current system SOC.

Change-Id: I58214402b2d79feb25209f5a1451d3f41edf181b
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>