xilinx axi ethernet: allocate memory for buffers.

The xae(9) driver expects that the "memory-region" property is always
provided in its DTS node, but this is not a case for every platform.

If the property is not provided, then no restriction on buffer location
is in place and busdma backend could be used.

Since the Xilinx AXI DMA engine driver does not support busdma(9), then
allocate some memory for buffers manually, so we don't panic.

This fixes operation on Codasip A730.