liteeth: Add little endian target support

[occheung]

Summary

This patch adds Ethernet support to little endian targets (VexRiscv in particular).

Details

• Added little endian support to EthMac (with reference to LiteEth), to applicable targets, i.e.
  • afc3v1.py
  • kasli.py
  • kc705.py
  • metlino.py
  • mlabs_video.py
  • sayma_amc.py
  • simple.py