power: qpnp-fg-gen4: Add support to configure cutoff gain

Add a device tree property to configure the cutoff gain. This
configuration allows to tune how rapidly the unusable charge
is allowed to change.

Change-Id: Id31f29aa05afd6c3bce340b85c19bcd727b8b316
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>