Replace Xilinx Ethernet driver with lowrisc Ethernet driver

Created by: jrrk

This is the patched version of buildroot and friends which supports lowRISC Ethernet derived from the Alex Forencich implementation. At the moment only transmission is working. This could be due to incorrect hooking up to the interrupt sub-system, as you mentioned earlier. nevertheless you may want to merge and review with respect to your PLIC implementation. As far as I can see when running on the FPGA it does not hurt the rest of the Linux functionality. The ariane pull request also needs to be merged simultaneously or before this one to provide access to the updated device tree and associated boot rom. Meanwhile as it is inconvenient to produce Ethernet regression tests in simulation, I suggest hooking up the Ethernet and its PLIC interface to an Internal Logic Analyser instance - could be rewarding to see what can be achieved that way.