power: qpnp-fg: configure charger termination current

If the charger is using the fuel gauge ADC current to determine end of
charge, the termination current should be configured in the fuel gauge
SRAM register (0x4F8, 2) to make it work properly. Allow this register
to be configured via the devicetree property qcom,fg-chg-iterm-ma.

CRs-Fixed: 804096
Change-Id: Idc04c4d6e7cbd70c8e6d3d56789337667cfdb6d7
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>