arm64: dts: imx8mm: Add thermal zone support

Add thermal zone and tmu node to support i.MX8MM thermal
driver, ONLY cpu thermal zone is supported, and cpu cooling
is also added.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>