tg3: add support for Ethernet core in bcm4785

The BCM4785 or sometimes named BMC4705 is a Broadcom SoC which a
Gigabit 5750 Ethernet core. The core is connected via PCI with the rest
of the SoC, but it uses some extension.

This core does not use a firmware or an eeprom.

Some devices only have a switch which supports 100MBit/s, this
currently does not work with this driver.

This patch was original written by Michael Buesch <m@bues.ch> and is in
OpenWrt for some years now.

This was tested on a Linksys WRT610N V1 and older versions of this patch
were tested by other people on different devices.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>