power: qpnp-fg: add device tree support for batt-therm coefficients

The fuel gauge uses some coefficients to do battery thermistor
conversions. These values can vary depending on the thermistor and board
characteristics.

Add support for a device tree property qcom,thermal-coefficients to the
fuel gauge driver to allow users to configure these values.

CRs-Fixed: 713392
Change-Id: Ief2f75cbd5df28245baccdde9960f30d197d413c
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>