arm: dts: imx8mn: protect the firmware/optee node with ifdef

There is no need to include the firmware/optee node if the optee
driver is not enabled.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>
diff --git a/arch/arm/dts/imx8mn-u-boot.dtsi b/arch/arm/dts/imx8mn-u-boot.dtsi
index bc57566..5e07055 100644
--- a/arch/arm/dts/imx8mn-u-boot.dtsi
+++ b/arch/arm/dts/imx8mn-u-boot.dtsi
@@ -8,12 +8,14 @@
 		multiple-images;
 	};
 
+#ifdef CONFIG_OPTEE
 	firmware {
 		optee {
 			compatible = "linaro,optee-tz";
 			method = "smc";
 		};
 	};
+#endif
 
 	wdt-reboot {
 		compatible = "wdt-reboot";