drivers: thermal: tsens: Create function to return sign-extended temperature

Hide the details of how to convert values read from TSENS HW to mCelsius
behind a function. All versions of the IP can be supported as a result.

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/0689917475cf83b7e01f6978504fd37352a5e3ca.1572526427.git.amit.kucheria@linaro.org