power: qpnp-fg-gen4: Add maintenance charging support

Maintenance charging is to show the battery state of charge (SOC)
monotonically for a better user experience by not allowing it to
drop by a big margin when there is a discharging post charge
termination and SOC is held at 100%. Add support for it.

Change-Id: Ia85b7b739eb5372f82fd273462702f8445ef7d28
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>