thermal: imx_tmu: Check the TEMP range for iMX8MM

On iMX8MM, the V flag in TRISTR register only reflect the state of SNSR
value, not the calibrated TEMP value. So checking this flag is not
reliable. Per IC suggestion, change to read the TEMP/AVG_TEMP directly
and check whether it in valid range 10-125C.

Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>