ARM: dts: imx6ul: Add missing #thermal-sensor-cells to tempmon

Fixes the dtbs_check warning:
  tempmon: '#thermal-sensor-cells' is a required property
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/nxp/imx/imx6ul.dtsi

@@ -638,6 +638,7 @@ tempmon: tempmon {
 					nvmem-cells = <&tempmon_calib>, <&tempmon_temp_grade>;
 					nvmem-cell-names = "calib", "temp_grade";
 					clocks = <&clks IMX6UL_CLK_PLL3_USB_OTG>;
+					#thermal-sensor-cells = <0>;
 				};
 			};