drivers: imx_tmu: Select polling-rate from cpu-thermal devicetree node

The polling rate is already specified in some devicetrees, like
imx8mp.dtsi for example, but was not selected so far. For the
trippoints, the cpu-thermal node is used. Also get the polling rate from
this node. Use the default of 5000ms if the polling rate should not be
specified in the devicetree.

NOTE: The polling rate from the devicetree will be used after this
patch. In imx8*.dtsi devicetrees the polling delay is set to 2000ms for
example.

Signed-off-by: Benjamin Hahn <B.Hahn@phytec.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>