thermal: hisilicon: increase temperature resolution

When calculate temperature, old code firstly do division and then
convert to "millicelsius" unit. This will lose resolution and only can
read back temperature with "Celsius" unit.

So firstly scale step value to "millicelsius" and then do division, so
finally we can increase resolution for temperature value. Also refine
the calculation from temperature value to step value.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>