ARM64: dts: amlogic: gxbb: Enable secure monitor

Add the secure monitor node in the Amlogic Meson GXBB DTSI file to
enable it.
Signed-off-by: Carlo Caione <carlo@endlessm.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>

arch/arm64/boot/dts/amlogic/meson-gxbb.dtsi

@@ -102,6 +102,12 @@ psci {
 		method = "smc";
 	};
 
+	firmware {
+		sm: secure-monitor {
+			compatible = "amlogic,meson-gxbb-sm";
+		};
+	};
+
 	timer {
 		compatible = "arm,armv8-timer";
 		interrupts = <GIC_PPI 13