ARM: dts: zynq: Fix memory size on the Zybo Z7 board

According to the reference manual, the board has two Micron
MT41K256M16HA-125 DDR3L memory ICs, which have 512 MiB each

Tested on a ZYBO-Z7-20 board
Signed-off-by: Luis Araneda <luaraneda@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm/boot/dts/zynq-zybo-z7.dts

@@ -13,7 +13,7 @@ aliases {
 
 	memory@0 {
 		device_type = "memory";
-		reg = <0x0 0x20000000>;
+		reg = <0x0 0x40000000>;
 	};
 
 	chosen {