tg3: Create MII_TG3_FET namespace

Broadcom's phys come in two distinctly different register layouts.  For
the lack of an official term to distinguish between the two formats, we
can loosely categorize them by their fast ethernet or gigabit ethernet
transceiver description.  This patch creates the (driver-internal) Fast
Ethernet Transceiver (FET) namespace and converts the 5906 EPHY
definitions over.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>