qpnp-fg-gen3: Backup the learned capacity during capacity learning

When the capacity learning algorithm completes, we store the
learned capacity to BattCapActual word in FG SRAM. Though this
is correct and needed for coulomb counter to operate, value is
not retained across reboot. Backup the learned capacity to a
scratchpad SRAM word (address 74) to overcome this.

Change-Id: I39c257876776a9e9c872add1366569394c49ef01
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>