ARM: dts: at91: sama5d2_xplained: Add memory node in devicetree

sama5d2_xplained DRAM detection code will be modified to use device tree
instead of hardcoded addresses. In order to prepare that, add the memory
node to at91-sama5d2_xplained.dts.

Signed-off-by: Clément Léger <clement.leger@bootlin.com>

arch/arm/dts/at91-sama5d2_xplained.dts

@@ -12,6 +12,10 @@
 		stdout-path = &uart1;
 	};
 
+	memory {
+		reg = <0x20000000 0x20000000>;
+	};
+
 	onewire_tm: onewire {
 		gpios = <&pioA PIN_PB0 GPIO_ACTIVE_HIGH>;
 		pinctrl-names = "default";