net: axi_ethernet: Add driver to u-boot

Add axi_ethernet driver for little-endian Microblaze.

RX/TX BDs and rxframe buffer are shared among all axi_ethernet MACs.
Only one MAC can work in one time.

Signed-off-by: Michal Simek <monstr@monstr.eu>
Acked-by: Mike Frysinger <vapier@gentoo.org>