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>