[TG3]: Add basic 5906 support.

Add support for the new 5709 device.  This is a new 10/100 Mbps chip.
The mailbox access and firmware interface are quite different from
all other tg3 chips.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>