sfc: Add back mailing list

We used to have a mailing list in the MAINTAINERS file, but removed this
when we became part of Xilinx as it stopped working.
Now inside AMD we have the list again. Add it back so patches will be seen
by all sfc developers.

Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>