net-next: stmmac: Add dwmac-sun8i

The dwmac-sun8i is a heavy hacked version of stmmac hardware by
allwinner.
In fact the only common part is the descriptor management and the first
register function.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>