power: qpnp-fg: configure cutoff voltage via devicetree

Expose the fuel gauge cutoff voltage in the devicetree, so that users
can configure the zero point of the battery model.

CRs-Fixed: 771380
Change-Id: Ie20509bc5900a636c5624bdea0df04b394ee1c32
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>