power: qpnp-fg-gen4: Read battery temperature from FG SRAM

Currently, battery temperature is read from RR_ADC peripheral
which is having less resolution (1 C) than the value obtained
from FG SRAM (0.25 C). Switch to read it from FG SRAM for better
resolution.

Change-Id: I72befe1e0d01841707c6ec0df39d78a2509c53b3
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>