thermal/drivers/hisi: Simplify the temperature/step computation

The step and the base temperature are fixed values, we can simplify the
computation by converting the base temperature to milli celsius and use a
pre-computed step value. That saves us a lot of mult + div for nothing at
runtime.

Take also the opportunity to change the function names to be consistent with
the rest of the code.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Leo Yan <leo.yan@linaro.org>
Tested-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>