tg3: Add AC131 power down support

The AC131 does not respect the power down bit (bit 11) of the MII
Control Register (reg 0x0).  Instead, software is required to put the
phy into standby power down mode through the shadow register set.  This
patch implements support for the AC131 standby power down mode.

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