arm64: dts: meson-axg: add secure monitor

Add the secure monitor device to the axg platform.
With this, we can read the SoC serial number.
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>

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

@@ -110,6 +110,10 @@ l2: l2-cache0 {
 		};
 	};
 
+	sm: secure-monitor {
+		compatible = "amlogic,meson-gxbb-sm";
+	};
+
 	psci {
 		compatible = "arm,psci-1.0";
 		method = "smc";