Add device driver support for the Broadcom BCM570x family of gigabit

Add device driver support for the Broadcom BCM570x family of gigabit
ethernet controllers. This adds support for the 3Com 3c996-T, the
SysKonnect SK-9D21 and SK-9D41, and the built-in gigE NICs on
Dell PowerEdge 2550 servers. The latter configuration hauls ass:
preliminary measurements show TCP speeds of over 900Mbps using
only normal size frames.

TCP/IP checksum offload, jumbo frames and VLAN tag insertion/stripping
are supported, as well as interrupt moderation.

Still need to fix autonegotiation support for 1000baseSX NICs, but
beyond that, driver is pretty solid.