thermal: rcar_gen3_thermal: Update temperature conversion method

Update the formula to calculate temperature:
Currently, current TEMP is calculated as
average of val1 (is calculated by formula 1)
and val2 (is calculated by formula 2). But,
as description in HWM (chapter 10A.3.1.2 Normal Mode.)

If (TEMP_CODE < THCODE2[11:0]) CTEMP value should be val1.
If (TEMP_CODE > THCODE2[11:0]) CTEMP value should be val2.

Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>