net: ethernet: xilinx: Do not use axienet on PPC

Axi ethernet can't be used on PPC because it is
little endian IP and PPC is big endian.
This system can't be designed.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>