thermal/drivers/tsens: Support using nvmem cells for msm8974 calibration

MSM8974 has two sets of calibration data: main one and backup. Add
support for parsing both sets of calibration data from nvmem cells.

Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>