DM9000: Add support for DM9000A and DM9000B chips

Add support for both the DM9000A and DM9000B versions of
the DM9000 networking chip. This includes adding support
for the Link-Change IRQ which is used instead of polling
the PHY every 2 seconds.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>