zynq: Change memory node name from memory@0 to just memory

The reason is that memory@0 doesn't overwrite memory node taken from
skeleton.dtsi and final DTB contains two memory nodes. Both of them are
used and empty one is ignored by Linux but it should be overwritten by
by memory node from platform dts.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>