arm64: dts: imx8ulp: add thermal node

Add thermal node. Cooling map is not added, because frequency runtime
changing not supported for now.

Reviewed-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>