ARM: dts: Rename memory@0 to memory

zynq-7000.dtsi include skeleton.dtsi which contains memory node with
base address and size zero. If you add memory@0 node to the platform DTS
in final DTB there are two memory nodes and U-Boot works with the first
one (with zeros) which end up in failing in dram_init because size is
zero.
Platform memory node should rewrite default memory node setup from
skeleton.dtsi that's why platfroms needs to also use memory as node name
instead of memory@0.

Reported-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>