arm64: dts: imx8mp: Add thermal zones support

i.MX8MP has a TMU inside which supports two thermal zones, add support
for them.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>