zynq: add memory nodes to device tree to initialize DRAM with OF

zynq: add memory nodes to device tree to initialize DRAM with OF

Commit 9e0e37ac added OF RAM initialization support
but memory nodes are missing in device tree.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Tested-by: Masahiro Yamada <yamada.m@jp.panasonic.com> [on ZC706 board]
Tested-by: Michal Simek <michal.simek@xilinx.com> [on ZC702 board]
Signed-off-by: Michal Simek <michal.simek@xilinx.com>