power: qpnp-fg-gen4: Add support for rapid SOC reduction

In certain cases, battery state of charge (SOC) can stay higher
than 0 when the battery voltage drops below the cutoff voltage.
Userspace initiates a shutdown only when SOC hits 0. To avoid a
potential under voltage lockout and help with graceful shutdown,
add support for rapid SOC reduction on detecting this condition.

Change-Id: I9a23e67afee68060bd1127699205973a61c3d09d
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>