ARM: dts: imxrt1020-evk: add device_type = "memory" to memory node

Now device_type = "memory" is mandatory to allow u-boot to read memory
node, so let's add it to memory node.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>

arch/arm/dts/imxrt1020-evk.dts

@@ -20,6 +20,7 @@
 	};
 
 	memory {
+		device_type = "memory";
 		reg = <0x80000000 0x2000000>;
 	};
 };