imx8m: fixup thermal trips

Fixup thermal trips in Linux device tree according to SoC thermal
grade.

Signed-off-by: Andrejs Cainikovs <andrejs.cainikovs@toradex.com>
Reviewed-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Tested-by: Adam Ford <aford173@gmail.com>