cpu: imx8: use intended cpu-thermal device when getting temp value

This fixes getting DT alert and critical pdata values in imx_scu_thermal
driver. On i.MX8QXP using not initialized alert pdata value resulted in
boot hang and endless loop outputting:
CPU Temperature (47200C) has beyond alert (0C), close to critical (0C) waiting...

While at it, preset CPU type values once to avoid multiple calls
of device_is_compatible() for same property.

Fixes: 3ee6ea44 ("cpu: imx_cpu: Print the CPU temperature for iMX8QM A72")
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>