dt-bindings: imx-thermal: Add #thermal-sensor-cells property

This property is defined in thermal-sensor.yaml. Reference this file and
constraint '#thermal-sensor-cells' to 0 for imx-thermal.
Fixes the warning:
arch/arm/boot/dts/nxp/imx/imx6q-mba6a.dtb: tempmon:
 '#thermal-sensor-cells' does not match any of the regexes: 'pinctrl-[0-9]+'
 From schema: Documentation/devicetree/bindings/thermal/imx-thermal.yaml

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20231012080033.2715241-2-alexander.stein@ew.tq-group.com