tg3: Add 5720 NVRAM decoding

The 5720 implements its own NVRAM pin strapping scheme.  This patch adds
the required support.

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