thermal/drivers/hisi: Fix configuration register setting

The TEMP0_CFG configuration register contains different field to set up the
temperature controller. However in the code, nothing prevents a setup to
overwrite the previous one: eg. writing the hdak value overwrites the sensor
selection, the sensor selection overwrites the hdak value.

In order to prevent such thing, use a regmap-like mechanism by reading the
value before, set the corresponding bits and write the result.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>