Add a low-level driver for Mellanox MT23108 and MT25208 HCAs.  The
MT25208 is only fully supported when in MT23108 compatibility mode;
only the very beginnings of support for native MT25208 mode (required
for HCAs without local memory) is present.

(As a side note, I believe this driver would be the first in-tree
consumer of the PCI MSI/MSI-X API)
Signed-off-by: Roland Dreier <roland@topspin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>