net: Add driver for LiteX's LiteETH network interface

LiteX is a soft system-on-chip that targets FPGAs. LiteETH is a basic
network device that is commonly used in LiteX designs.

The driver was first written in 2017 and has been maintained by the
LiteX community in various trees. Thank you to all who have contributed.

Co-developed-by: Gabriel Somlo <gsomlo@gmail.com>
Co-developed-by: David Shah <dave@ds0.me>
Co-developed-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Tested-by: Gabriel Somlo <gsomlo@gmail.com>
Reviewed-by: Gabriel Somlo <gsomlo@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>