drivers: ethernet: Add Xilinx AXI Enet driver

The Xilinx AXI Ethernet subsystem is commonly found in FPGA designs.
This patch adds a driver and device tree bindings for the Ethernet MAC
core and its MDIO controller.
The driver was tested on a RISC-V softcore in an FPGA design, with an
RGMII phy and Ethernet subsystem version 7.2 Rev. 14. Device tree
bindings match the device tree generated by Vitis hsi. Note that Vitis
generates one of the two included compatible strings depending on
version.

Signed-off-by: Eric Ackermann <eric.ackermann@cispa.de>