dt-bindings: thermal: imx: update the binding to new method

Due to the old method has already been marked as deprecated in binding
doc, so obviously it's better to update the example to new bindings
as well.

Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Leonard Crestez <leonard.crestez@nxp.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>