thermal/drivers/tsens: Replace custom 8960 apis with generic apis

Rework calibrate function to use common function. Derive the offset from
a missing hardcoded slope table and the data from the nvmem calib
efuses.
Drop custom get_temp function and use generic api.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Acked-by: Thara Gopinath <thara.gopinath@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210420183343.2272-7-ansuelsmth@gmail.com