net: ethernet: Add a driver for Gemini gigabit ethernet

The Gemini ethernet has been around for years as an out-of-tree
patch used with the NAS boxen and routers built on StorLink
SL3512 and SL3516, later Storm Semiconductor, later Cortina
Systems. These ASICs are still being deployed and brand new
off-the-shelf systems using it can easily be acquired.

The full name of the IP block is "Net Engine and Gigabit
Ethernet MAC" commonly just called "GMAC".

The hardware block contains a common TCP Offload Enginer (TOE)
that can be used by both MACs. The current driver does not use
it.

Cc: Tobias Waldvogel <tobias.waldvogel@gmail.com>
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>