drivers: thermal: tsens: simplify get_temp_tsens_v2 routine

The current implementation is based on an algorithm published in the
docs. Instead of reading the temperature thrice w/o any explanation,
improve the algorithm.

This will become the basis for a common get_temp routine in the future.

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>